Completed Sleeve edge too tight

Hi everyone, I wonder if you have any ideas to help me out of my predicament. On completing a childs cardigan with shawl collar I discovered that the cast on stitch row of one sleeve is quite tight, is there any way of rectifying this without having to take it all apart !! thank you.

Can you undo cast on row, pick up the stitches and then cast off loosely? Most cardigans jumpers end up with cuff turned back. I might be tempted to use a bigger needle for the cast off.