Correct. There're a lot of tiny snippets of information around that upgrading the F-2 to the MG 151/20 was done on a great scale without any official documentation. For example the very first F-2 the RAF captured (F-2 of I./JG 26 in early 1941) already had the 15mm MG 151 replaced by the 20mm MG 151/20. Also known is a report of a F-2 which was used to test the RZ-65 rocket installation which is also listed as having a 20mm cannon.
The technical differences between MG 151 and MG 151/20 are very small and exchanging the vital parts could be done in short time (i.e. during one of the regular engine checks). Initially it seems as if that depended a bit on the pilot (someone "important" would get the parts easier and faster than a nameless pilot) while later it seems to have been a broadly used modification, especially on refurbished aircraft (such as those given to JG 51 in mid-1942).
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