Repot in spring, preferably early to mid rather than late spring. Regarding the potting mix, probably a mixture of ericaceous and John Innes No. 3 would be the best. As to how to do it, make sure the plants are well watered in their current pots, remove from pot and position in the new one with however much compost mix in the bottom to achieve a more or less similar planting depth, then pack the compost mixture in round the sides, making sure you push it down to exclude air pockets. Water thoroughly when finished (this assumes there are drainage holes in the pot, which there should be) and recheck the level of the compost (it sometimes sinks too low after watering, leaving the top of the rootball exposed). Position somewhere sheltered, out of strong winds and preferably out of reach of sunlight in summer between 11 and 3 p.m.
One other thing - if the roots are wound round and round the base of the root ball, you may need to tease these out so they're sticking out a bit more to enable easier penetration into the new compost medium.

id be inclined if you can ofcourse to put them in the ground .

Acers should always be transplanted in the winter months when they are dormant, and quickly enough that they don't realise they have been moved, so don't wait until the spring. Nosey Potter is right that they do best planted out, but if you have to move them to a bigger planter then do it now, on any frost free day to a larger pot. As bamboo says tease the roots out. Plant in ericaceous compost or tree/shrub and rose compost, either in the pot or outside. water in well. Keep moist, but not drowned. Sharp frosts may curl and kill outer leaves, so put in a protected part of patio/garden if you can.

I stand by the advice I gave above, despite Avkg47's differing opinion, which, as far as I'm concerned, certainly applies to 'transplanting', meaning moving from one piece of open ground to another - but not for pot culture. There will be feed in the new compost you use around the rootball in larger pots - this will be useful to the plant in spring, but not in winter.

I agree to differ on timing, bamboo. We have all tried various things that work best for our own gardens, but may work differently for others. I personally prefer not to disturb plants once their sap is rising, rootlets growing etc, but would rather give them a good start by re-potting etc over winter, as long as the weather is reasonably mild so roots are not burnt off by the cold before they establish themselves, and they can then take off straight away in the spring. It is a method that works for me with most pots and plants. There are of course others that you can only do in spring, summer or autumn. It is my preference and works well for me.

i am lucky enough to have a friend and nieghber whos quit the bonzie expert . you have to windows of opertunity with plusses and minesess to both ie early winter or late winter thow saying that when i wanted a willow to put my ashes under i just cut some new growth of mid winter on a mild day . planted it about 3 winters ago as a twig near a river . this is now dangling in the river and as thick as my bicep at least .

Oh so its still a twig then? Lol xx

very funny you know im 6`3" skillen or should i say shorty lol x x x.

Hello NP - just noticed your logo changed from viking to thistle ... been thinking something was different for a while now, but I am steam-powered in this age of technology - brain and computer!

well its related to a thistle but its a globe artichoke . change is as good as a rest as they say x .

Yes, I see that now, NP - blush! Relating to the original question, perhaps bamboo and I are not so far apart ... the weather is so mild at the moment, and have noticed my own acers are budding, so although it is January, if Marjorie's acers have buds, they can be re-potted any time now, if her weather is also mild?! Of course, a sudden late hard frost always burns the buds or any new leaf, regardless of re-potting now or in february/march.

i totaly agree ya got to go with nature even if it is cruel sometimes avq47 x .

Tell me about it ... I have today been looking at roses budding and thinking that when I cut them down in early march (if the current mild weather continues) I will be cutting all their flower heads off, but ... as you say ... cruel to be kind ... splindly roses are not pretty, so I shall have to just take a deep breath and prune!! Though if february is mild too, I might just cut them down sooner, then feed them up lots and lots in late spring! Comments and thoughts appreciated.

if they dont come good this year they should next . be posative .