Welcome to the board and to DH diving.
The answer is pretty simple, you can upgrade your 2nd or 3rd DH reg, it's a disease and no one ends up with just one.
To some extent I am kiddng but truth is most of us end up with several.
To answer your question, almost everything you use to service or upgrade a DA
is reused when adding a Phoenix. This is true wheather you do a like original rebuild or go whole hog with a HPR, new diaphragm and a duckbill eliminator. There is one inexpensive gasket and a filter that is not reused plus any internal parts in the HP nozzle that you choose to replace. The total cost of those is under $25. If (or more likely when) you get a second reg, the rebuilt nozzle can be installed in the second reg if you like. The same holds true if you were to upgrade it with a Cyclone nozzle.
If you choose to hold off on the Phoenix, I would suggest you at least install the HPR and the single stage main diaphragm that is recommended for the HPR (which can not be used with the original second stage). They are easier to install and adjust than the original equipment and will make a huge improvement in performance of the reg. The 2 stage version (with the tabs)of the replacement diaphragm
will work with the HPR or the original second stage but there is no need for it with the HPR. Replacing the main diapragm is pretty much a requirement reguardless of how the original looks. The originals are vastly inferior to the VDH silicone ones and while the origial will operate with the HPR, it is a huge loss in performance if you do.
The Duckbill eliminator can be installed and used with any combination you choose or the original type duckbill will work with any combination as well.
The one accessory that you may not have considered that I highly recommend for any DH (unless you are doing an original restore) is a band clamp. Those C clips are a PITA and discourage you from inspecting the internal area of the reg on a regular basis.
While ordering parts, I would also suggest you get a hookah port adapter. You will need some way of checking/setting IP on the reg both after the rebuild as well as for perodic testing over the years. With the adapter, you can connect any LP hose accessory like an IP guage plus it can be used for an octo or LP inflator.