I think W34/70 is very flexible and you won’t have any issues with it. I’ve done something similar just out of laziness or brewing too late in the spring when the temperature in the brewery no longer drops below 18C. (I usually brew lagers in the winter and early spring when I can keep the brewery closer to 10-12C and then I just turn the heat up for a diacetyl rest.)
Typically after 10 to 14 days, you won’t get off flavour from warming your lager. In fact, you’ll be doing your beer a favour by having the yeast break down diacetyl (if present, not all lager yeasts produce it) and cleaning up the beer a little more. Long term rest is probably best done at much lower temperature though. My diacetyl rests are usually just 2 to 3 days.
Some lager yeast produce more diacetyl than others, so I’ve just take the habit of doing pretty much the same thing time after time regardless of what yeast I’m using. That is, very similar to what you do but with a shorter rest at higher temps.