You've got a couple of options I think:
Change the TAGs individually at the registars and then merge the domains from your Nominet account
or
Merge the lot at Nominet first and then pay £10 to do a bulk TAG change
If your registrant details are similar but different at each registrar then you'll need to either get Nom to amend them so that they can be merged or hold them in 2 different batches.
They will keep the nameservers.
Grant