Where are you at?

Shouldn't need to get it re-complianced here in NSW. It only matters if it went from petrol to LPG.

You're going to get a whole whopping load of opinions on this but you can fit the lot and have it checked off to be certified if it wasn't already. They just need to check your work but some will say no as they just want the work for themselves.
Even when making the rebate claim, it is advised to write yourself a receipt for the labour you put into installing the LPG system so they can see who did it and a dollar value placed against it. Which is just s**t on a piece of paper because who charges themselves but that was what they wanted.

But like I said, you are going to get a dozen different of opinions and each will strongly disagree with each other. Mainly probably disagree with what I said. But if there is any doubt in yourself, then don't even put a spanner to, leave it for someone else.