Your conundrum lies in matching mass flow. With cap tube metered setups, it is very important to match the compressor to the cap tube to the heat exchangers. They all effect each other. Whenever some piece of the system changes, guess what? Cap tube has to change length. Swapping from one size of compressor to another, cap tube has to flow different.
How much different? We can't tell you. It's a balancing act. Too much flow and you will either flood your compressor or not get enough head pressure. Too little flow and you will have gobs of superheat but not much mass flow. Somewhere in the middle lies max performance. To make matters more difficult, when you change the cap tube, the optimum system charge changes with it.
Since your new rig (working, I hope) is set up for R-410A, that's what it will get max btu output using for refrigerant. My guess is that you will be able to do more than 15000 BTU for the first little while, due to the huge loop and water source. Much like the water heater, performance will drop off after initial start-up. For this size, a TXV (made for R410A) is worth it. The extra mass of gas the compressor can move (if it is allowed to) adds up in droves of extra heat transfer.
The downsides are many. Like you said, you will need to change the oil. Not only will you have to drain your oil tank, but the plumbing must be flushed as well. Double or triple flushing is better. The filters will need new ones. R410 is not cheap or as forgiving as propane, either. It runs at much higher pressure across the board, so your tubing and connections need to be stronger. POE oil absorbs moisture and won't let go of it. Me no likey R-410A. Too picky and pricey for me.
If you run propane, the system will not develop a full 15000 BTU of heat transfer. More like the 11000 that the last one was rated at, maybe a full ton. I would probably run a TXV (made for r22/R502) on it, rated around 1 ton. If the system can do more heat transfer, the TXV can open up to supply the extra gas.
The underlying question here is: How much heat do you really want to feed the greenhouse? If the unit works, will it be a good match for your heat load? If it is too large, it will short cycle and waste lots of energy starting and ramping up with each cycle.
I would definitely rig it up and play with it for a while to see how it does while it is above freezing outside. You can install it backwards and fool the thermometer to make it heat, or just vent the cold air outdoors with the whole rest of the unit indoors. Running on low fan, it won't suck too much air in from elsewhere.
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