Upvote:3
Let's talk hypotheticals and say there was no pandemic, that's how I will approach this question as the pandemic introduces too much uncertainty.
On September 5 2019 I moved to Austria for PhD studies. My passport allows me to enter Schengen Area with no visa for 90 days. To stay and study, I needed a D-Visa and was issued one at the Austrian representation in my country. BUT, it was issued for the start of my studies (Sept. 20. 2019). Which meant that I would arrive in Austria without an as-of-yet valid visa and was entering using my visa-free tourist privileges. The Austrian consulate insisted there was no issue and that the border guards see that all the time.
Fast-forward to Vienna Airport: I land and walk toward border control, palms sweating like crazy, hand my passport over and he sees the visa which will be valid 15 days from then and asks me simple questions and lets me pass. I had no issues AND I did not leave when my visa became valid as there is no need to do so.
Since then, I have received a working permit and have left the Schengen Area once and reentered with absolutely no issues.
Short answer: As long as you do not overstay your 90 visa-free days before your visa comes into effect, there is no problem. If you ever hit 90 days before your visa is valid then you must leave.
This is not legal advice. I am simply sharing my own first-hand experience in a similar situation