I think a lot of the characteristics that I consider inherently Bertie -- generous spirit, ability to overlook selfishness and lack of thoughtfulness in others, dislike of cruelty and bullying, interest in popular entertainment and fashions, quite bright in a very unfocused way -- would remain. These aren't qualities defined by his station since most of them aren't shared by his peers and family. So it stands to reason that he'd still have them, he'd just be used to being a relatively unseen servant and have become used to the expectation that he *shouldn't* show his personality so clearly.

(So, basically, I think Bertie-as-servant would act much like Bertie-as-nephew: while he knows he shouldn't say some things, he'd probably spend a lot of time blurting things and reacting badly, and being forced to apologise for it.)