Hello!

I guess most (all) of the distro's you are talking about use GRUB (or
at least they allow to do it). If that's true, I'm pretty sure you can
happily let them overwrite the GRUB in MBR as many times as they want,
since it's the same (or just probably minor version differences)
bootloader. Just make a copy of /boot/grub/grub.cfg and make sure it's
the same on every partition. Or, even better, if that's possible right
now, make a common /boot partition and after installing the new distro
just merge the (probably new) /boot/grub/grub.cfg with your old one.

I really think that *should* work!

Take care,
ks. Wojciech Kuzyszyn