This is only a little change but you know, baby steps :)

I want to branch out from the Crofter jumper by using other yarns (still DK though), I know with some times there is an issue with tension with some yarns but how easy is it just to swap brands just to make a standard one colour jumper?

If you have a similar yarn and you have checked that the yardage is approximately equal then that's a good start. The next step is to do a tension square using the correct size needles and see how it compares to the stated tension using the recommended yarn.

If the tension comes out the same as the pattern then you've got a green light. If it's too small then do another tension square with one size larger needles, too large then do a square with one size smaller needles.

Once you've matched the tension then fire away :)

Medusa has pretty much hit the nail on the head there Moonfire.