From the PHB, p 28:
Power: You can replace a power with another
power of the same type (at-will attack power, encounter
attack power, daily attack power, or utility power),
of the same level or lower, and from the same class.
And from the page before it:
At 13th, 17th, 23rd, and 27th levels, you can
replace any encounter attack power you know from
your class with a new one of your new level (or an
encounter attack power of a lower level, if you choose).
The first one lets us power-swap through retraining the powers gotten from multiclass feats, provided the new power is from the same class as the old. Alright. Good to know. It doesn't care if that power is from multi-class feats or from the Half-Elf's dilettante ability (so you can retrain that, so long as you don't change classes).
The Second point however hinges on one simple question.... if you take multiclass feats, or even the multiclass paragon path, or -even- a paragon path of the multiclass.... does this allow you to swap using your power swap at levels 13, 17, 23, or 27 (for encounter powers)?
A character who has taken a class-specific multiclass
feat counts as a member of that class for the purpose of
meeting prerequisites for taking other feats and qualifying
for paragon paths.
So then.... what does this mean? Well, it means that as it stands, you can't use multiclassing to give you the right to swap powers out at levels 13, 15, 17, 19, 23, 25, 27, or 29. All a multiclass feat allows you to do is use that feat to substitute for the class as a prerequisite for paragon paths and for feats. That's all.
'But, Dracosuave, with the awesome badmusic referencing name, how do I get high level abilities with these feats?'
Simple.
Any time you gain a level, you can alter that
decision. Effectively, pretend you’re choosing the
power-swap feat for the first time at the new level
you’ve just gained. You gain back the power that you
gave up originally from your primary class, lose the
power that you chose from your second class, and
make the trade again. You give up a different power
from your primary class and replace it with a new
power of the same level from your second class.
This is -seperate- from the retraining option. This allows you to update your second-class powers, and is the only way to do so. This will not permit you to update your paragon path powers, as those are set in stone, and not affected by your multiclass feats.
However, unlike with a paragon path, you don't have to make the choices for what those extra abilities are until you gain those levels. They are not set in stone for you, and some of those encounter and daily and utility abilities those classes get might be a better match than a paragon path that's more designed in tune with the class features of that particular class.
Again, flexibility in design costs you the rigid number of powers you'd get otherwise. This makes it so that one choice is not automaticly better than the other for all builds. This is good design.