Ply diffrerences

I found a pattern for a baby blanket but it says the yarn is 10 ply. I wanted to knit in a finer yarn for a more heirloom effect. Will it affect the size much if I do so from 10 ply to 2 ply?

There’s so much variation in ply size that it’s no longer a good gauge of yarn weight.

It’s much better to use the stitches/inch or tension as an estimate of yarn weight.
That said, there’s probably enough difference between a 2- and a 10-ply to have a major effect on the size of the finished blanket. Better to look for a pattern that uses 2-ply to begin with.
What is the gauge or tension given for the pattern and for the yarn that you want to substitute?

18 stitches = 4 inches