So let's see if this is about how it goes?
Level 1: Take Imp. Unarmed Strike
Level 3/5/7: Take Feats that required Imp. Unarmed Strike
Level 8: Take a level of monk and gain "Unarmed Strike"
Now you want to retrain out your level 1 feat since you meet the pre-requisites? However, your character I thought had to be 'legal' at all levels. Since you didn't take monk until level 8, your feats that require Improved Unarmed Strike would have to be taken after that in order for you to meet the pre-requisites.
John