Verizon iPhones come SIM-unlocked for GSM use, so you should be able to just get some T-Mobile SIMs and go to town. Getting the SIM cards from T-Mobile's web site sometimes has better deals (free or 99¢) than the $15/SIM they charge at most stores. You'll get them in the mail with a generic number and then you can port your number with a phone call - it's easy as long as you have all your Verizon billing info handy.
Here's Verizon's unlock info:
http://www.verizon.com/about/responsibility/product/device-unlocking-policy
"We do not lock our 4G LTE devices, and no code is needed to program them for use with another carrier."
The only other issue to think about is LTE band support. For the iPhone 6s, it will support all the T-Mobile LTE bands. For the 6 and 5s, the Verizon model is the same as the AT&T/T-Mobile model. The 5 may be the trouble spot, as the Verizon model (A1429) only supports bands 1,3,5,13,25 and the AT&T/T-Mobile version supports 2,4,5,17. T-Mobile tends to operate on bands 2 and 4, with 12 being rolled out in some areas. For that, your 5 may only get you HSPA+ "3G/4G" speeds and you might want to look at selling/trading them for something with better T-Mobile support.
You can read more about LTE bands here:
http://www.apple.com/iphone/LTE/
Band 12 seems to be the "big" upgrade for T-Mobile, as it penetrates further in rural areas and buildings (it's 700MHz and branded as "Extended Range LTE"). It's not available in all areas, but does make a difference where it is - when I was with T-Mobile on an iPhone 6, saw maybe a dot of service in some basement areas at work, while a colleague with a 6s saw 4 dots. Unlike T-Mobile, the older iPhones supported AT&T's (17) and Verizon's (13) 700MHz bands, as they had been rolled out for some time. Thus, if you want to use an iPhone on T-Mobile and get the best results everywhere, something like a 6s or SE has the ability to pick up more towers than the older models. Still, my experience with T-Mobile on a 5s/6 was pretty good, especially with Wi-Fi calling as a fallback. If you're not in a market that supports band 12, it really doesn't matter:
http://www.tmonews.com/700mhz-lte-map/
By the way, in terms of iPhones, anything with a SIM slot is GSM-compatible.