It's quite possible the result would be less seats. A straight swap of votes between the parties is far from guaranteed, and not standing makes it formal, so they lose their individuality.
On the plus side, that is more significant for Reform and the Tories. While they have plenty of followers who would vote for either (or a merged party), they both have a significant number of voters who would never vote for the other party. Polling suggests that's MUCH more pronounced than with the Lab/LD/Green voters.
I suspect that Reform may not have anywhere that they can easily improve electorally. They've taken the obvious voters from the other parties, but would have to identify as more left or right wing, in order to make significant further inroads, and they're likely to lose voters from the opposite side if they try too hard.
