Here's my question. can I stretch the 7 to a size 8 to avoid
re-creating a wax master in size 8?
If you are making only one, or just a few multiples, absolutely, go
ahead and stretch it.
If you plan to make hundreds, make new models and rubber molds for
each size and you can just stretch to make the half sizes.
It will vary with the design, but what you have to ask yourself is,
in time or money what does sizing the rings cost compared to making a
new mold. If you are making 10 rings size 8 maybe you are better off
with a new mold. If only one, probably not.