Replaced my hot end … now have banding

Hello. I’m new to this forum but I am here reaching out for advice. I recently swapped in a new hot end and put my printer to work using a filament I have used extensively … I noticed early on with the first layer that it was banded, striped, it would print a few rows thickly and the next rows thinly and then thick again …

Before I changed the hot end the layers were lovely and even.

any ideas what might have happened and what I might be able to do to resolve this?

thanks in advance.