McLaren will be bringing upgrades in the next couple of races - this was confirmed by Seidl. They're probably bringing more than the other midfield teams at least.
While the MCL35 will be an evolution that doesn't mean there won't be non-trivial changes. For example, they might well have a rather different FW, which will of course have side-effects on the rest of the car, some parts more than others. So the parts of the car they're likely bringing upgrades to now are likely parts that they're pretty sure won't need to change next year or can be upgraded with relatively little R&D.
While the regulations aren't changing next year they're changing massively in 2021. Which likely means all (or most) teams will be entirely focused on the 2021 car from early on in 2020. Putting it another way, most teams will probably bring only minor upgrades after winter testing and probably only to the first 5 or so races. Which means that, right now in 2019, if you want to start a R&D project to upgrade a particular part and it's estimated to require 6 months, then you have to be starting it right now (at the latest) otherwise it's too late.
You might ask, well, what if doing that work would provide a big benefit (getting 0.1s from a single part would probably be a lot), wouldn't it be better to do it anyway? The problem is that 2021 is a total reset. Meaning, most R&D on the 2020 won't be useful after 2020. Some parts will be largely unchanged, though until the regulations are finalised it's hard to be sure. The PU regulations aren't likely to change much so that region will probably be fairly stable and I'd imagine that a very tight engine cover will still matter in 2021, but perhaps the optimal shape will change a lot. In the end, what this means is any R&D for 2020 will only get a payoff for one season, but any R&D for 2021 will likely get a payoff across many seasons. Which means that it's actually quite rational to treat 2020 as something of a "throw away" season. Obviously, no team would intentionally harm their chances but what will likely happen in practice is that most teams will simply have to accept that their relative performance at the start of the year will likely be quite close to their relative performance at the end of the year and that the absolute performance of the cars won't change much. Those fighting for the championship might do something else, but for 7-8 teams, this is likely to be the case. In turn, this means that the current R&D work for the 2020 cars is critical - if it goes badly, you might as well write off your whole year. Nobody will even consider doing a B-spec car in 2020. Conversely, if 2020 development goes well then you're likely to have a very good year with little worry about others catching you.