No - it is not possible for "rootstock" blooms to suddenly show up on old canes which previously produced blooms of the scion (desired top rose). Rootstock blooms will always show up on their own canes, whether old or new.

However it is possible that the "old" canes with the single pink blooms were there before, but not yet blooming, so you did not notice them. In that case they could be rootstock.

Please post a picture of the single pink blooms - someone may be able to identify them.

The other thing which might have happened is that your rose sported (mutated) to this new bloom type, but again the canes which are now "old" would have existed in prior seasons, possibly not yet blooming. I believe that sports can happen on new growth off old canes, but to take over half of the bush it seems to me that you would have noticed them before.

Please post a pic of the entire bush - this will be interesting to everyone!

Jackie

There is another possibility called a graft-chimera, but is very unlikely.