I learned in C152 and C172 and was able to convert to a PA28 in less than one hour.
Whether or not I could have gone the other way I don't know, as I can't go back and start over.
However, the reason I'm replying is I'd love to know where all these jobs are that are in C152/172's as written in the OP. All I see are unemployed CPL/ATPL holders.