Should I have the return line replaced when installing a new heat pump?
The size really would not be a problem, the 410 is a higher pressure system and that allows for smaller tubing but will take more refrigerant (not much).
I am not a fan of reusing parts, yes they can be cleaned but I have had friends come to me after a new retrofit failed in less than 2 years.  The cause of the failure new Poe oil mixed with mineral oil that was not cleaned completely. This has happened 2x that I know of , is the evaporator rated for the 410a pressures?
I would make sure they will warranty all work like I said the size won’t be a problem but will they use that to get out of a warranty claim?  Both my friends had systems retrofitted by hvac companies they only knew they cooled the house. Both companies used the existing equipment as the reason for failure , it won’t affect the system but if you re use anything make sure the system is covered by the warranty as installed.
To cover the system they may want to replace the line, don’t let that be a reason for them to deny warranty coverage but it would not be a problem in theory.