Shouldnt have to worry.
Depends what engine thay are refitting. It should be a brand new unit with 0 miles so will have to be run in and taken steady for a few miles.
Apart from that i think it will be a good think as you will have a warantee with the new engine and all work carried out.
Good luck mate and welcome to the club
Yeah its a brand new one through the warranty agreement...so I am assuming it will be a 0 mile engine. Yeah i'll ease the new one in slowly...as much as I have missed my little car!!