A fact to consider:
California regulators are actively talking about regulating reefer emissions as they do trucks. In their view, it would be just fine to have two reefer fleets (all carriers combined) in the nation, one running in CA with "clean" reefers, and all the rest that would not be permitted to run in CA.
While this is prompting understandable howls of protest from interstate commerce quarters, and while CA's draconian approach to reefer emissions my be restrained by federal laws, CA has fired a significant shot across the bow of anyone running reefer trucks.
No matter how CA's reefer-emissions laws change, and they likely will, a new reefer would likely be grandfathered in and would have a longer useful life. If you take your old reefer with you to a new truck it's usefulness may be limited, at least in CA.
Also, if you have not already done so, check out new reefer features and prices. Great strides have been made in recent years. Reefers are not the same as they used to be.
Going new gives you the opportunity to revisit the under-body v. top mount decision. You have under-body reefer experience already. Some drivers point out that under-body is a severe reefer-operating environment. Up top, you are free of the road spray, salt and exhaust heat that sometimes contribute to reefer failures.
Compared to top-mounts, under-body mounting adds $3,500 to $5,000 to the cost of a reefer installation. The difference is due to the labor rate being charged and the skill (speed) of the technician. Those that have done a number of split reefer installs can get the job done quicker.
Under-body mounts have their advantages, the main one being it creates more room for sleeper space if you are partial to a large sleeper.
If you are running with FedEx, a new reefer would open you to their new T-val freight, for which shippers pay a higher tariff, and which would give you additional loads to haul - an important consideration if slow economic times return. Among drivers and fleet owners, the jury is still out on the value of equipping a truck for T-val. Some say it's good, others say it is not. Note that if FedEx ever reduces it's fleet size, T-val equipped trucks would likely be the last to go.
Of course, the cost of moving equipment from one truck to another will enter in to your decision. I would be very interested, as would others, to hear what that cost would be. I hope you'd be willing to share here the estimates you receive.