Now. Whilst it is dormant, certainly before March. Take as large a rootball as you can round the roots and after transplanting keep moist for a couple of months. A handful of superphosphate and some compost in the new hole would help too unless you are an organic grower in which case use the compost plus whatever organic growers use for root development. Always dodgy moving mature trees but this one is still small.

Plumb tree, love it. Hope you get it in upright! Couldn't resist, sorry. Worthy

Thanks for the advice even about keeping it upright