You can switch to any network but if your existing contract hasn't expired yet, you'll be charged if you want to cancel.
And there's no guarantee that Verizon network will be any better for a good signal in the same area, so ask others who use Verizon before switching.
I have a nexus 5. I have straight talk. And my carrier is AT&T. But the connection is not very good. Can I switch to Verizon with straight talk.?
Best place to check is with Straight Talk. If you want to use a Verizon tower, it won't work with your existing phone, you will need to get a new phone.