Yes, you would need to change the spocket if you changed the pitch of the chain and bar.
On a small saw of 2.0 hp or so a 3/8 pitch Lo profile is usually used.
A saw of around 2.7 to 4.0 hp usually has .325 pitch from the factory but on the upper end of this range you might see a regular 3/8 pitch being used.
Tell us what saw you are referring to and we could help you more.
I don't own one of these but I would say it has a 3/8 Lo Profile Chain.
It probably has safety links in the chain for Low-Kick back.
I think you should stay with the 3/8 Lo Pro but getting a chain that is not a Low-Kick back will make the saw cut faster.
A Oregon 91VX is good, but I like Carltons 3/8 Lo Pro the best.
If all this is confusing go to a saw dealer and ask for a 3/8 Lo Pro chain that doesn't have Low-Kick back links. It's a little more dangerous so be careful of kick back.