As a regular in the U135 section, I would welcome changing this to being FIDE rated for the reasons you give. Increasingly, it appears that many of those who are eligible for the lowest section, play up a section (presumably because of the FIDE rating or for the reasons I have for considering switching, given below). A consequence is that in the lowest section you have a polarisation of entrants between stronger players (who may have a chance of winning something) and beginners. I therefore find that I tend to bounce between competitive and rather easy games - for that reason I was considering playing in the section above to ensure I get five testing games over the weekend. As most of my club and league chess is not FIDE rated, FIDE rating in itself is not a reason for switching. Also, I'm a Gold member anyway, since the London Chess Classic and many sections of the British Championships require it, so cost is not an issue.
I'm not sure what the answer is to your question about what grades to use - presumably if you define what date's grade you will be using, only a fairly small number of people will be affected (and most of those will just switch section, rather than being excluded).