Just got a pair of these off a bike from Canada so they are front brake on left etc. Can I swap the hoses without having to bld the system?

If your carefull.

yes, easily as SSS said – be careful.

just dont squeeze the brake lever by accident with the hose disconnected.

Probably if you enlist some help to cover all the open bits with fingers, I’d do it from the caliper end as the threads are easier to restart putting it back together.

I should probably mention that my Julies had a barbed push hose fitting and plastic hose as opposed to the nut/olive connection of higher end Maguras..

Thus, you may find you need to remove the calipers from the bike as undoing the connection at the master cylinder/lever end might just end up twisting the hose as you undo the nut.
If you’ve got decent braided hose with nut/olive hardware, jobs a good un and you can do the swap with everything in place.