So back to the question. Can I or can I not flash a stock 'rn' file over an 'lp' ecu? Simple question, only requires a 'yes' or a 'no'...
I don't have an LP file at hand, but the general rule is same bootrom will crossflash. Run ME7check on both and note the bootrom. Both are likely 5.12 which would make it possible.
If they are 5.12, you would be well served to note what Carsinc posted, namely the HS/HN option. -Adam posted his HN and a pretty good .kp/.xdf for it. Even if you start from scratch I would switch to a well defined SW for a less painful experience anyhow...