ARC only affects the way you hear your mix
during mixing. Just think of it as transporting your DAW to an almost perfect room and mixing your track there. The whole point of mixing in an acoustically treated room is to enable you to make accurate mix decisions which will translate well on other systems. When you mix in an untreated room, your mix decisions are influenced by the room. If your room is boosting 100Hz, you're going to cut that frequency in your mix. Played in a room which doesn't boost 100Hz, that cut is going to sound wrong. When you mix in a good room (or with ARC) you can be sure that your mix decisions are based on the music, not the room.
Your old pre-ARC projects were mixed to sound good in your room. Whether they translated well to other systems is probably hit and miss. If you remix them with ARC enabled, you can be sure they will sound great on other systems too. Although it's still a good idea to listen to your mixes in other environments to make sure.
Don't worry about what happens when you export your track with ARC turned off. As long as you set up ARC correctly (i.e. followed the instructions to the letter and did at least 12-16 measurements in a symmetrical pattern around your listening area) and as long as you have good quality monitors, you can be confident that your mix decisions were not influenced by the room like they were before. Once you have an exported mix, listen to it without ARC and compare it to your reference mixes. Without ARC, the room will be coloring the sound, but hopefully in the same way as it's coloring the pro mixes you're referencing.