I don't expect Brennan to be every bit as good in LF, or even close really.
Brennan does have +4 OAA in RF (his best position) over 994.2 innings. Kwan has +22 OAA in 2654.2 innings of LF, his best position. Neither have put up good numbers in the other OF positions they have played, probably mainly because they are settled into their sections in the OF so they are not perfectly calibrated to jump into other fields and play those positions as well.
I do think Kwan is a slightly better OF than Brennan but if I am to have concerns it's more going to be that Brennan just hasn't had much chance to take on a new position (AFAIK he's almost entirely been a CF/RF in his career). So if he's only to play the position for a few weeks I don't expect it to go extremely well, just like Kwan didn't do well in RF for reasons outside of his weaker arm.
No need to panic, sure, just I do expect a downgrade and a pretty significant one with the gap gradually closing over time but no I don't think Brennan will ever be equal.