Any suggestions for perennials the would flower early and, most
important,
will not mind being moved into a pot whilst the Aster takes its place and
does its stuff.


Not quite what you ask for, but you could try early flowering biennials
such as wallflowers (Erysimum cheiri), honesty (Lunaria annua) or
forget-me-not (Myosotis). And that leads onto the possibility of bulbs.
I'd guess that you could treat narcissi or tulips in the desired manner.

I grow a rhizomatous Sidalcea (I think it's Sidalcea candida), which
seems to lift and divide fairly readily, but admittedly I've mostly done
that in spring, and the rhizome I moved in late autumn 'cos it was
growing into some other plants didn't take well to being moved that
late.
--
Stewart Robert Hinsley

Thanks for your help Stewart - the forget-me-nots do muscle their way in
anyway - but they do a useful job.