How do I move a hot-water convector?
If you are not in the Southern hemisphere, wait 4-6 months and do this in the off-season for heating, not in mid-winter.
It's major surgery and requires shutting down and partially (at least) draining your boiler, unless somebody put in WAY more valves than is usual.

Shut down the boiler
Turn off the water supply to the boiler, if it's not nomally kept closed.
Drain system to remove water (or potentially antifreeze) from this portion of the piping (at
least.)
Cut or unsolder pipe to release convector, and remove it, assuming no convenient unions.
cut or unsolder more pipe to undo what leads here.
assemble pipe to lead to new location, taking care not to drill holes in things you should not, or set the building on fire while soldering.
If you have two sections joined by elbows, yes you can straighten the bend in the plumbing by replacing those with straight couplings. Your outer cover may no longer fit properly, however, since it was configured for the corner.
connect the new plumbing to the newly configured convector in the new location.
pressurize the system and check carefully for leaks
bleed air from the system - may need to repeat several times after the next step, as air dissolved in cold water comes out of solution in heated water.
restart the boiler
balance the system for heat delivery, using balancing valves, movable flaps on the convector (not that the style you show appears to have those) or system zoning. Installing balancing valves and/or zoning would have to be dealt with at the &quot;replumbing&quot; stage if not already present.