NEVER use soil in pots ... use potting media ...

sand is irrelevant.. if you water enough ...and perhaps use a little fert in early or mid summer ...

you are over thinking this... they should have already been moved ..

if they regress next year.. move them back ...

ken

ps: if it was horrible hard packed clay.. then you ought to be thinking about it .... the issue with soil types is drainage ... if no draining.. roots rot ... if too much draining.. then water more ... but do not amend the planting hole.. the sand will leach all the water out of the peat ... and you will end up worse than native sand ...