You can always cancel your upgrade later (subject to availability of the same fare bucket originally purchased) at no cost. So definitely go for it now, and if it looks extremely full then cancel.

Even in peak periods some flights can be really full (weekends) whereas others can be just short of full. I have the worst luck on earth so my flights usually have a couple of spare seats. I did fly CDG-DXB a couple of years back in mid-July - Y was packed but nowhere near oversold, and J was only half full.