Greg and Kelly Carr wrote:

I took apart an existing wall in my home and found that 15 amp outlets were
used on a 20 amp circuit. Should these be replaced with 20 amp?



No, this is specifically permitted in the (US) Code.
Any 20 Amp circuit with more than one recept may have
a mix of 15 and 20 Amp recepts. Art. 210-21 and 210-24

Jim