It seems like there's a little cross-traffic here between the two standard mechanics for swapping around powers while you are leveling up: Retraining powers, and Replacing powers.
Retraining allows you to replace a feat, skill, or power once per level. For powers, it says "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" (PHB p28) So you could not use retraining to replace the power you got from MCing into Psion with a higher level Psion power, as it would need to be "of the the same level or lower".
Replacing allows you to, at certain levels, replace lower level powers with higher level ones. However, the text of power replacement for encounter powers says "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)" (PHB p27) While this does allow you to 'upgrade' a lower-level encounter power for a higher level one, it specifies "power you know from your class". And taking a class-specific multiclass feat only allows you to count as a member of that new class for the purpose of meeting prerequisites for taking other feats and qualifying for paragon paths; it doesn't make that new class "your class" for power replacing. So this also does not allow upgrading of the power acquired from the Psion MC feat.