An awful lot has been written about the Commission rifle, mostly wrong.
First, it is not a Mauser. But it certainly shows the ancestry of the M 71, which was a Mauser. It was also influenced by the Mannlicher system and used a five round en-bloc clip which entered the action and dropped out the bottom when empty (unlike the top ejection of the M1 (Garand) rifle's clip).
Then there is the much discussed matter of barrel inside diameter. The orignal dimensions were a .311' (7.9 mm) bore and a .318" (8.08 mm) groove diameter. But the Germans found that such low lands wore out quickly in use due to the combination of corrosive primers, hot smokeless powder and steel jacketed bullets. So, in 1905, shortly after a new rifle (the Model 98) was adopted, they adopted a new cartridge with a larger diameter bullet and deepened the grooves to .323". Bore diameter remained .311", giving the new rifling a depth of .006", deep enough to prevent the problems with the earlier shallow rifling. (Groove depth of a U.S. .30 rifle is .004, giving a groove diameter and bullet diameter of .308")
Model 98 rifles were recalled and rebarrelled. But that was not done with the millions of Model 88's still in reserve service. The Germans found that firing the new bullets through the old barrels did not create a severe pressure problem, but the fact that the new case neck had nowhere to expand did.
So the Germans (not the Turks) modified millions of Model 88's to use the new "stripper" clips, ran a new reamer into the chamber to expand the chamber neck, stamped S on the receiver to show that the rifle could be used with the new "S" bullet, and re-issued the rifles to the reserves.
The rifles were not rebarrelled, the barrels were not rebored or re-rifled.
Ammunition confusion was not an issue; the converted rifles could not use the old ammo, which was in en-bloc clips, and the unconverted rifles could not use the new ammo, which was in stripper clips.
So, can a converted Model 88 be safely fired with standard WWII 7.9 ammunition or equivalent? A qualified yes, but those rifles, after all, are at least 114 years old and have been used, abused, worked over, and worked on by several generations of armorers, gunsmiths, wannabe gunsmiths, hobbyists, and bubbas. So, I have to advise caution.