noob ? about unlocking T-Mobile phone

OK, I know next to nothing about getting phones unlocked. Anyway, my contract w/ T-Mobile is just about over (next week). I have a 1 yr old Razr V3T that is in great shape and a tad better than the regular razr my sister has w/ sprint. I plan to get a new phone now, so, if I get my razr unlocked can she use it w/ her sprint plan by just exchanging sim cards? Will the swapped out sim cards be transparent to the phones and/or sprint?

What other use will my old phone be good for (sans door jam jokes)?