admin renewal with membership type change does not respect selected record

if a contact has multiple membership records and you initiate a renewal from the admin side, and then change the membership type, the first current membership record in the listing is changed – not the record for which you actually initiated the renewal. this is because we are unsetting the membership record ID if the membership type changes, and we shouldn't.

to reproduce:

1) create two membership records with different membership types for a contact
2) for the second record, click more > renew
3) change the membership type to a different type
4) notice that the first membership record was renewed – not the record you initiated the renewal for

see attached patch for the fix.