"Rebuilding" [Player's Handbook II, page 197], in summary, allows a character to change his class levels. By RAW, these levels can be replaced with any other class level as long as his remaining levels (those not being rebuilt) would allow him to qualify.
Because of this, characters can play with only prestige class levels if desired in many cases.
Any class that has a Base Attack Bonus requirement and advances Base Attack Bonus by +1 per level can be rebuilt. For example, if a class requires BAB +4, you must first qualify and then enter it. Since it advances BAB by +1 at first level, you have BAB +5. If you rebuild a class level, you only count your remaining BAB (+4). Thus, you still qualify for the BAB requirement. You then have three base class levels and two prestige class levels. You can repeat this until you no longer have base class levels.
This same process applies to classes requiring Caster Levels that fully advance caster level at each level with one exception: you must retain at least 1 level of the base class for any prestige class that adds to your progression of a base class (as, without a base class, it cannot be added to anything and thus ceases to function).
Additionally, this also works with skill requirements. Since skill ranks are independent of class levels, you can rebuild new prestige class levels as long as you are already qualified for the prestige class in terms of skill ranks.
The following example uses both Rebuilding and Retraining (both located in the Player's Handbook II).
Below is a step-by-step method for minimizing the requirements to enter Ruby Knight Vindicator. "=>" indicates retraining a feat selection when it is no longer required or rebuilding a class level when it is no longer required. Soldier of Light allows you to rebuild your cleric level (and thus recover your base attack bonus) because it will fill the requirement for Ruby Knight Vindicator and thus you will not lose the benefits of the prestige class.
Step 1: Cleric 1/Fighter 1/Warblade 1
Human: Favored
1: Primary Contact
Fighter 1: Martial Study
3: Martial Stance
Step 2: Cleric 1/Fighter 1/Warblade 1/Ruby Knight Vindicator 1
Human: Favored
1: Primary Contact => ?
Fighter 1: Martial Study
3: Martial Stance
Here you need to use skill point retraining to retrain a point into Knowledge (religion) at level 4 just before you enter Ruby Knight Vindicator. This will allow you to enter a level early (in addition to the cityscape feats, which let you enter another level earlier).
Step 3: Cleric 1/Fighter 1/Warblade 1/Ruby Knight Vindicator 2
Human: Favored => ?
1: ?
Fighter 1: Martial Study
3: Martial Stance
Step 4: Cleric 1/Fighter 1/Warblade 1/Ruby Knight Vindicator 3
Human: ?
1: ?
Fighter 1: Martial Study
3: Martial Stance => ?
6: ?
Step 5: Cleric 1/Fighter 1=> Something 1/Warblade 1/Ruby Knight Vindicator 4
Human: ?
1: ?
Fighter 1: Martial Study => ?
3: ?
6: ?
Step 6: Cleric 1/Something 1/Warblade 1/Ruby Knight Vindicator 5
Human: ?
1: ?
3: ?
6: ?
Step 7: Cleric 1/Something 1/Warblade 1/Ruby Knight Vindicator 6
Human: ?
1: ?
3: ?
6: ?
9: ?
Step 8: Cleric 1/Something 1/Warblade 1/Ruby Knight Vindicator 7
Human: ?
1: ?
3: ?
6: ?
9: ?
Step 9: Cleric 1=>Something 1/Something 1/Warblade 1/Ruby Knight Vindicator 7/Soldier of Light 1
Human: ?
1: ?
3: ?
6: ?
9: ?
Alternatively, you can use an Elf and take Fighter 2 to gain the feats you need in time. Then, by forgoing taking Soldier of Light (losing a single point of base attack bonus to Cleric 1), you can enter Eternal Blade and at level 20 you will have access to Island in Time (Eternal Blade 10). Now you can take your turn as an immediate action and then you and your cohort (identically constructed; alternatively, your cohort can forgo the Eternal Blade levels since only one of you needs Island in Time in most situations and your cohort will gain it only once you're in epic levels anyway) can use White Raven Tactics with one another.
As you can see, Rebuilding allows you to excise undesired levels that are otherwise needed to qualify for prestige classes.
I hope this has been helpful.
I will update this post with some of the popular examples from the old board soon.
