For both sides, while upgrading takes a back seat to perfecting routines during the comp season, if a child has the routines well in hand, some time is generally devoted to working new skills or skills that aren't yet in the routines in periods between meets. My DD is currently working on a new beam acro (her season will end in mid-March) and my DS still has a few skills he'd very much like to get into his routines this season before the end. Some of this for him will depend on whether his season ends in early April or in May. For him, he has been putting a little more time into the "almost within reach" skills and less time into the "maybe next year by states if it all goes well" skills as we move toward the high-stakes part of the season. As an XCel Diamond, my DD has 10.0s on every event but would like to upgrade next year for fun. My son moves to L10 next year, so he will for sure need to upgrade his dismounts on most events and hopes to add at least some difficulty on all.
You'll find that as they move along, the skills get harder and it takes a longer window of opportunity to get them, make them consistent, and get them into routines. While most gyms will do most of the intensive skill work in the off season, it doesn't make sense for optionals to confine it entirely there. My son, for instance, has been working on early drills since last fall for the Tkachev on high bar that he would love to compete next year, but probably won't until 2019-20.