Hey,

I'm changing the heads on my car to the mivec ones. Apart from the head gaskets will I need to replace anything else? Like the head bolts?

I'm pretty sure there torqued and will need new ones regardless.
Its a good idea as they are under alot of stress

The manual says torque em down, back em all off and re-torque them. To me that's going to stretch the bolts. I'll go buy a set just to be sure.

Just checking because at work we dont use the same bolts when we replace the heads on the locomotive engines. Lol there's at least one head change a week.

They are most likely to be "Torque to yield" bolts.
This means in the factory the torque machine is monitoring the applied torque against bolt rotation. When yield occurs in the boolt the relationship between torque applied and rotation changes.

For this reason it is definitely better to replace the bolts.
If you buy a Mivec head gasket set with steel head gaskets from a decent supplier in the USA the extra cost for a set of bolts isn't much.
Gerry