As you have unlinked the old tree it would be best now to work on the new one and keep that synched with FTM and Ancestry.
You could delete the old one but make sure that all info is on the newly synched one. I lost all the info that I had in my 'notes' sections on FTM for some strange reason the last time I had to resynch but had kept the old tree on FTM and have been gradually restoring the info onto the new one ... (with accompanying moans and mutterings against synching)
You will need to re-invite all of the invites on the old tree to the new one :-0
You could delete the old one but make sure that all info is on the newly synched one. I lost all the info that I had in my 'notes' sections on FTM for some strange reason the last time I had to resynch but had kept the old tree on FTM and have been gradually restoring the info onto the new one ... (with accompanying moans and mutterings against synching)
You will need to re-invite all of the invites on the old tree to the new one :-0