Dave,
Depending on if the 'old' vertical covered 40 meters at
all, the 'new' vertical might be an improvement. But not
really 'much' of one, there may be no practical difference
in performance on 40 meters.
Would 15 meters be a bonus? If you mean an unexpected
improvement over the 'old' vertical, then, no, the 'old'
vertical probably would work better on 15 meters.
If you don't have the "bunch of in-ground radials" already
in the ground, put them in and then put the 'old' vertical over
them! I think you'll see a greater over-all improvement that
way, rather than with just the 'new' vertical (except on
40 meters, maybe).
"Full sized" antennas work 'better' than 'shortened'
antennas,
in general, but sometimes that 'better' is sort'a hard to hear.
'Doc