Will concede to the more learned contributor on this question.
Personally do not use TM Backup but another form of backup namely Clone and perform this on a daily basis.
If as mentioned earlier and it can not be done -->>
Suggest getting a new Erie with at least 2 1/2 Greater Drive Capacity that the Internal Drive Capacity.
Then put aside the existing TM Drive for safe keeping and start a New TM Backup regime on the new drive.
This would provide access to the Old Data and still have the protection on the new drive.
Users' choice for what best works for their setup