Are they gonna pay for this 'sex'? <g>
The problem is: You could do it, as part of the whole 'warm & fuzzy, keep the customer happy bit', BUT
This sexy-ness is NOT defined and you could spend expensive hours tweaking colors,button placements etc.

My advice?
Approach this cosmetic makeover like a full blown project, totally analysing and determining the user requierments and produce an design specification document that they will sign off on.

give 'em screenshots and a printout of the bitmaps directory in Foxpro and have them specify what they want. (PaintBrush Pro is good for that http://www.jasc.com/ ) That should take 15 minutes, time you can give away,hopefully.

Tell them it should take maybe 1/2 to 1hr per screen
AND maybe 1.5 hours "overhead" to rebuild /retest the app.

If they still wan't to go ahead,then sure!
And now, you have the documentation (additional services request) to get paid for it, and, if you already have been paid an enormous sum and have no more space in your bank account, do an invoice and make it no-charge/complimentary etc.

hope this helps!