How to size down

[TxCynDoll]

Ok, I can't seem to find many cardigans for 0 to 3 mos and style for boys...I don't want to do just the st st style...this is what I want to do, I want to take a pattern I have for a 6 month old and size it down to a 0 to 3 mos...my new grandson is a week old and if I make it for 6 mos, it will be to hot here in Texas...it calls for sport weight yarn and size 4 and 5 needles...I want to use Bernat softee baby and maybe go down on needle size...My question is, will this work? Any options or suggestion would be helpful...

Thank you in advance...