i'd agree - this is where the fascinating subject of studio psychology and politics merge recklessly!
for a client i know well, i'll always admit that there might be things that i would 'do better now'. especially if i have the excuse of a new bit of kit or something similar. this often happens in long projects. i worked with an artist for over a year on one project, and in the meantime i changed all my mics and pre's and was just really up front and told her some of her early vocals i reckoned i could do a much better job on. she was delighted as she'd been worried about asking me if she could re-record some of her early stuff cos she wasn't happy with it! sometimes these things work out.
i did make a botch a while back, and again, i was upfront with the client, and used the 'i really want to do the best job i can - and i'm not going to charge you for the extra hour' and again, they were quite happy with that.
most folk want to hear themselves at their best, and with a bit of diplomacy, you can achieve a lot. i like that this studio lark is not just about gear!
interestingly, i had almost the opposite problem a while ago in that i had some work brought in that had been tracked elsewhere and there was sooooooo much wrong with it as the engineer thought she knew everything and that just by buying lots of expensive equipment, everything would sound great first time (which naffed me off anyway for hugely devaluing our craft anyway) - there is a recent post from me about one aspect, which was that a vocalist she'd used had a really really bad lisp and the song involved possibly more 'sssss' in the lyrics than i'd ever heard before.
everyone here was really helpful and understanding (what i love about this place) but recommended a few tech solutions (which went some way) - but in the end, i had to say something. the client and i have a frustrating relationship at the best of times, but this was going to be difficult. in the end, she now refers to the track with the 'lisp' she has accepted that this really was an issue, having denied point blank in the first place that this was the case.
more times than not - the truth is the best route - it's about that ole devil called spin!!!
i'd go down the route of saying that you think you could do a better job as you've learnt more about them as a band (assuming this is the first time you worked with them). i think that could work.
i've done that where we've been developing up from say just an
acoustic guitar part and vocal, to a full production number - sometimes, when you are just letting stuff develop, things WILL change, and you need to have that option at the end to rerecord stuff. i'm often quite up front about that at the beginning of working on that sort of material. it's not been an issue yet.
hope it goes okay - let us know.
very best wishes
paul d