The rules don't really work together
Let's start with retraining (PHBII page 191). It cannot change your class levels - for that, you need rebuilding (PHBII page 196).
However, you cannot rebuild a monster class, because Savage Species page 152 says:
A monster may not multiclass until it completes the full progression of its monster class.
As soon as you "trade out" any monster class levels, the only thing you can replace them with is the same levels you just gave away. So no, you cannot just dump RHD and put class levels in their place. You could use the rebuilding option to change your race, but that removes the benefits of your old race as well as its RHD and LA.
There are monster class rules in other sources, most notably the Savage Progressions archive on the Wizards of the Coast website. Those rules do let you multiclass, but this brings with it another issue - you lose the monster's abilities when you trade out the levels that gave them to you.
Since level adjustment buyoff only works on class levels, not RHD, you will have a lot of difficulty buying off LA if you can't get rid of RHD.
Once the total of a character's class levels (not including any Hit Dice from his creature type or his level adjustment) reaches three times his level adjustment, his level adjustment is eligible to be decreased by 1.
But there are other rules that can help you
Since monster class rules are unfriendly, we can ignore them entirely and just take a typical monster without a monster class, but with an LA. The monster then goes on a night out and meets a fellow succubus, who hits him with a bunch of negative levels. In 24 hours...
If a negative level is not removed before 24 hours have passed, the affected creature must attempt a Fortitude save ... On a failure, the negative level goes away, but the creature’s level is also reduced by one.
After becoming a creature with 1 RHD and however much LA, the monster embarks on an adventure to gain character levels the old-fashioned way. With many fewer RHD, it will also be easier to buy off level adjustment (though you can't buy off more than 3 LA before level 20).