necklines!

Hi - does anyone have experience of changing necklines on patterns? I’m knitting a sweater for my grand-daughter from a vintage knitting pattern (Hayfield Diane 875). It’s a rollneck pattern but I want to change to a regular neckline with buttons on one side. any ideas? thanks

Welcome to KnittingHelp!
Take a look at the neckline before the collar or neckband is added on. It may be that the dimensions are fine as is and that you can just add a short band or edging onto it. (I couldn’t find a photo of the pattern.) If the neckline is too high as written, just start the bind off and shaping earlier. I’m assuming you’re thinking of a crew neck?

For the button closure on one shoulder you can stop the shoulder a bit short of the bind off and extend the front and back of the shoulder with a tab along the top edge. One tab has buttonholes. Take a look at some baby sweaters on Ravelry to see examples of this kind of closure.