Well you can follow as many gods as you like, and you will probably find that most people in the world follow multiple gods depending on the situation:
Is it winter? - better make a small prayer at the temple of the Raven Queen.
Am I going into battle? - better make an offering to Bane, or maybe Bahamut.
Am I betting my weekly wage on a dog fight? - better ask for Avandra to smile on me.
Am I planting a crop of wheat? - better make an offering at the shrine of Pelor.
So the general population will send prayers to a pantheon of gods, personal to them and dependant on season and their situation and outlook.
Adventurers are slightly different, but only due to the mechanics of faith or in other words actual benefits of following a god granted from magical items. For this reason an adventurer must name a dominant god, items attached to this diety will grant the additional benefits, other items attached to other gods will not.
Changing allegance to another god is possible in game, but for a divine class (who are the only ones who will get a benefit) it should be an important and definate situation. They should probably have either a defining moment where they switch allegance (which will seem obvious to everyone involved and will probably be for plot reasons) or should undertake a small quest or rite of passage, this little system will stop players from abusing the god system at whim whenever they find a better holy symbol.
In my campaign the Paladin of Erathis changed alligence to the Raven Queen when he was visited by a fallen angel of the Raven Queen who persuaded him to do it, tempting him with promises of increased power and offering him Raven Queen blessed (plot) items. This is an example of story based conversion that is fairly straight forward and natural.
Another example from my campaign is an elven ranger who started out following Melora, at epic he became a Harbinger of Doom and was initiated into a cult whose aim (it turned out) was to destroy the world to replace it with the far realms. He didn't really like that idea (and I don't blame him!) so he was approached by Sehanine who converted him to an order of hers that worked against the Harbingers of Doom. This is an example where a PCs choice of God has no real mechanical impact (ranger) so no quest was necessary and the plot weaved itself around his change of mind.
hmm I'm rambling a bit so I'll stop, but I hope this helps.