The cord in the awning fabric that slides into the slot extends past the fabric on both ends. You're supposed to fold it over into the roller tube then replace the end cap. That's supposed to keep the fabric centered on the tube. I did remove both caps when I replaced ours. The fabric we bought does not have a separate valance so once you have it secured to the roller tube in 2 slots, it's not going anywhere. So the question is can you get the old completely out with one end cap on?
The other issue you may run into depends on the replacement fabric. Not all 3 slots in the roller tube are the same size. We had to switch which 2 we used to tightly secure the awning to the tube. Since the left side end cap has a slot to install and remove the pull strap it needed to be removed and repositioned anyway. The right side end cap had to be removed so the lock lever ends up in the correct spot when the awning is closed. Seriously, they're aluminum head rivets and take about 30 seconds to remove. Having both caps off also made it easier to insert and center the awning. Just my opinion for what it's worth.