I am not sure about some of what you are saying. To the best of my knowledge, every M1 Garand manufactured, with the exceptions of early development rifles was chambered in 30-06. That includes the NM rifles built in the late 50s. Now if that is correct then any M1 Garand chambered in 7.62 NATO is a conversion. No original M1 Garand left a US arsenal (Springfield Armory) or contractor's factory ( Winchester WWII) or H&R (Harrington Richardson), IHC (International Harvester) post WWII chambered in 7.62 NATO. While in 1964 the Springfield Armory did a REPORT OF EVALUATION ON NAVY CONVERSION 0F RIFLE, U. S. CAL . 30, Ml TO FIRE 7 .62MM AMMUNITION BY MODIFICATION TO THE BARREL, I don't believe the conversions were actually done by the Springfield Armory. The report did pretty much show that a barrel sleeve was not a good idea and that the best approach was to use a barrel originally cut and chambered in 7.62 NATO. These conversions were also taking place long after the last M1 Garand was produced.
As to not converting a M1 Garand to .308 I see no reason not to. The rifles manufactured during WWII have a high collector value but only if the rifle is 100% correct as to each and every part. In the case of an early 100% correct rifle then yes, it would be foolish to convert a sought after collectable. However, the recent CMP rifles classified as "Correct Grade" were in the serial number range of 5.4 to 5.8 million making them well post Korea rifles starting around maybe 6/54. While a rifle like this may be correct as to parts these later rifles really don't command much of a premium as to collector value. Even their stocks at that point were the standard DoD cartouche. Rifles like these in my opinion are shooters rather than sought after collectables. The IHC and H&R rifles do command a slight premium of the later rifles simply because there were fewer produced. Again, even on these rifles they must be correct as to all parts to be of any value to a collector.
So in conclusion I see nothing wrong with converting a later 1950s rifle to the 7.62 NATO chambering. A serious collector is not likely to have much interest in the rifle anyway. If the rifle is to be enjoyed as a shooter it matters not in my opinion and granted just my opinion.
Just My Take
Ron