dogboy said:
Has anyone out there attempted to do this? I would like to convert some
because I believe that many Prestige Classes should have been Base Classes
to begin with, but someone probably got lazy and didn't want to detail all 20
levels. Any info would be appreciated.
You don't really need to
First of all, notice that the Prestige Classes you probably wish they were base classes, are by themselves quite generic. You mentioned the Assassin, and from the DMG the only others generic enough IMO would be the Blackguard, the Thaumaturgist and the Horizon Walker (ok, and the AT/EK/MT, but they are not prestigious in the original sense).
Making those characters concept a base class is very easy, but even easier is to implement them from an existing base class. For example:
Assassin
1) Be a Rogue: you get the same HD, BAB, ST, uncanny dodge, sneak attack and skills (even more actually).
2) Grant
Death Attack as a feat, and perhaps even require to spend skill points to raise the DC instead of being automatic.
3) Grant
Hide in Plain Sight as a feat, with high level requirements, or make it one of the Rogue's "special ability".
4) The save against poison can basically come from the Resist Poison feat, and
Poison Use can be a feat as well.
If you prefer, agree with the DM to exchange Rogue class features for those abilities instead of taking them as feats. For example, you could exchange all the trap abilities (trapfinding and trap sense) for Death Attack, or Evasion for the poison use and saves.
If you want spells, take a very few levels in a spellcasting class and choose spells oriented to the Assassin's job.
Blackguard
1) Be a Paladin, but switch everything Good with Evil and viceversa, and use the other spell list.
2) Exchange the Special Mount with Fiendish Servant (or keep an evil mount).
Thaumaturgist
1) Be a Cleric or Wizard or Sorcerer, or whatever you prefer.
2) Each special ability is worth very much a feat.
Such an implementation is even better, because this PrCl's special abilities are so few in number that they didn't need a PrCl at all, and furthermore the idea of being a summoner of this kind shouldn't be railroaded into the DMG specific implementation, which is only one of the possible.
Horizon Walker
1) Be a Ranger: you get the same HD, BAB, ST, and more skills.
2) Exhange favored enemies with terrain mastery: 1 at first level, and then every time you get a new fav.enemy + increase of previous one, you instead choose 2 new terrains.
! That said, if you want to have fun by converting PrCls into unique base classes, just do it

but my point here is that the PrCls which you may want to convert are often so generic and featureless that something very close is already doable with the core classes, just by turning special abilities into feats or exchange them for other class features. Other PrCl which are very different are also often the ones which have the most distinctive feel (and most powerful features) so it may be best to let them be prestige (and keep those feature only for high levels

).
YMMV