Don,
If I buy a new RTU L will it be compatible with the older unit when I get it back.
It's a TIU (Track Interface Unit), not an RTU. Yes, it will be completely compatible with your remote.
Are there any other issues that may come up with having the newer RTU L on the layout??
Yes. You will likely have improved DCS signal strength!
This and a whole lot more is all in "The DCS O Gauge Companion 2nd Edition", now available for purchase as an eBook or a printed book from MTH's web store site! Click on the link below to go to MTH's web page for the book!