They weren't converted
The US was making pattern 14 for the brits, needed more rifles and so put the P14 line into production making it in US 30.06, this isn't a .303 gun reamed out for 30.06, it's a gun that was made in BOTH .303 P14, and 30.06 M1917
Also the design has nothing to do with the British enfeilds, it's a Mauser action, basically it's the British Mauser, like the 1903 is the US Mauser (we paid them royalties all through WWI)