All reference to casting lower level spells with a higher level slot is removed from the class descriptions in 3.5 SRD. However I agree with Silveras. The entry specifically refers to "various character class tables" and refers to Any spellcaster.
Magic Overview 3.5 SRD:
"Spell Slots: The various character class tables show how many spells of each level a character can cast per day. These openings for daily spells are called spell slots. A spellcaster always has the option to fill a higher-level spell slot with a lower-level spell. A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his or her due still gets the slots but must fill them with spells of lower level."
Bard 3.5 SRD:
"Spells: A bard casts arcane spells, which are drawn from the bard spell list. He can cast any spell he knows without preparing it ahead of time. Every bard spell has a verbal component (singing, reciting, or music). To learn or cast a spell, a bard must have a Charisma score equal to at least 10 + the spell. The Difficulty Class for a saving throw against a bard’s spell is 10 + the spell level + the bard’s Charisma modifier.
Like other spellcasters, a bard can cast only a certain number of spells of each spell level per day. His base daily spell allotment is given on Table: The Bard. In addition, he receives bonus spells per day if he has a high Charisma score. When Table: Bard Spells Known indicates that the bard gets 0 spells per day of a given spell level, he gains only the bonus spells he would be entitled to based on his Charisma score for that spell level.
The bard’s selection of spells is extremely limited. A bard begins play knowing four 0-level spells of your choice. At most new bard levels, he gains one or more new spells, as indicated on Table: Bard Spells Known. (Unlike spells per day, the number of spells a bard knows is not affected by his Charisma score; the numbers on Table: Bard Spells Known are fixed.)
Upon reaching 5th level, and at every third bard level after that (8th, 11th, and so on), a bard can choose to learn a new spell in place of one he already knows. In effect, the bard “loses” the old spell in exchange for the new one. The new spell’s level must be the same as that of the spell being exchanged, and it must be at least two levels lower than the highest-level bard spell the bard can cast. A bard may swap only a single spell at any given level, and must choose whether or not to swap the spell at the same time that he gains new spells known for the level.
As noted above, a bard need not prepare his spells in advance. He can cast any spell he knows at any time, assuming he has not yet used up his allotment of spells per day for the spell’s level."
Sorcerer 3.5 SRD:
"Spells: A sorcerer casts arcane spells which are drawn primarily from the sorcerer/wizard spell list. He can cast any spell he knows without preparing it ahead of time, the way a wizard or a cleric must (see below).
To learn or cast a spell, a sorcerer must have a Charisma score equal to at least 10 + the spell level. The Difficulty Class for a saving throw against a sorcerer’s spell is 10 + the spell level + the sorcerer’s Charisma modifier.
Like other spellcasters, a sorcerer can cast only a certain number of spells of each spell level per day. His base daily spell allotment is given on Table: The Sorcerer. In addition, he receives bonus spells per day if he has a high Charisma score.
A sorcerer’s selection of spells is extremely limited. A sorcerer begins play knowing four 0-level spells and two 1st-level spells of your choice. At each new sorcerer level, he gains one or more new spells, as indicated on Table: Sorcerer Spells Known. (Unlike spells per day, the number of spells a sorcerer knows is not affected by his Charisma score; the numbers on Table: Sorcerer Spells Known are fixed.) These new spells can be common spells chosen from the sorcerer/wizard spell list, or they can be unusual spells that the sorcerer has gained some understanding of by study. The sorcerer can’t use this method of spell acquisition to learn spells at a faster rate, however.
Upon reaching 4th level, and at every even-numbered sorcerer level after that (6th, 8th, and so on), a sorcerer can choose to learn a new spell in place of one he already knows. In effect, the sorcerer “loses” the old spell in exchange for the new one. The new spell’s level must be the same as that of the spell being exchanged, and it must be at least two levels lower than the highest-level sorcerer spell the sorcerer can cast. A sorcerer may swap only a single spell at any given level, and must choose whether or not to swap the spell at the same time that he gains new spells known for the level.
Unlike a wizard or a cleric, a sorcerer need not prepare his spells in advance. He can cast any spell he knows at any time, assuming he has not yet used up his spells per day for that spell level. He does not have to decide ahead of time which spells he’ll cast."
Wizard 3.5 SRD:
"Spells: A wizard casts arcane spells which are drawn from the sorcerer/ wizard spell list. A wizard must choose and prepare her spells ahead of time (see below).
To learn, prepare, or cast a spell, the wizard must have an Intelligence score equal to at least 10 + the spell level. The Difficulty Class for a saving throw against a wizard’s spell is 10 + the spell level + the wizard’s Intelligence modifier.
Like other spellcasters, a wizard can cast only a certain number of spells of each spell level per day. Her base daily spell allotment is given on Table: The Wizard. In addition, she receives bonus spells per day if she has a high Intelligence score.
Unlike a bard or sorcerer, a wizard may know any number of spells. She must choose and prepare her spells ahead of time by getting a good night’s sleep and spending 1 hour studying her spellbook. While studying, the wizard decides which spells to prepare."