My understanding is that there is a fairly small number of pots
circuits (2?) that they can bring in over the B-PON, and that moreover
ISDN BRI and hicap (eg. repeatered or HDSL DS1 service) are entirely
incompatible.
In Virginia, there's anecdotal evidence that suggests that they'll
leave the copper upon request, and won't even try to remove it if you
still need it for service.
I can verify that they will leave it if you ask them to. At least in
Maryland. Though I told them to remove it.