mk2 seats into mk1

is it as simple as just change the runners or is there more to it?

they'll slide straight in but the fwd/bwd lever will be on the wrong side and won't locate in the spring loaded pin that locks the seat.

As above. If you want it to be a proper permanent job, there's a detailed 'How To' here :
http://www.vwgolfmk1.org.uk/modules.php?name=Forums&file=viewtopic&t=69930

iv got them in mine and like the other two guys said, they are fine but the adjuster doesnt work. but mine wobble a bit lol. theyre secure, but bit wobbly

i bought a set for my cabby they fitted straight in no changes whatso ever

post 1983 (i think) have the mk2 type runners