nothing is ever guaranteed 100% "safe" when you update software... there are just so many variables at play with different computers/configurations etc.
Having said that, if you simply shutdown v0.20.1... and then install v0.21 and then start v0.21... it should restart from where 0.20.1 left off... ie. at Block 235195.
Also, the initial sync shouldn't be taking 6 weeks to complete!!?!
What CPU/RAM combo are you using? Do you have an SSD or HDD? What is your download bandwidth? 
Having said that, if you simply shutdown v0.20.1... and then install v0.21 and then start v0.21... it should restart from where 0.20.1 left off... ie. at Block 235195.
Also, the initial sync shouldn't be taking 6 weeks to complete!!?!
What CPU/RAM combo are you using? Do you have an SSD or HDD? What is your download bandwidth? Thank you very mucht for your answer. It was because i saw here a topic of a person who got problems with v0.21 and not with v0.20.1.
My computeris 4 years old. Not a very fast one i think. Use windows 10. HDD 0.5 TB. But for the other like bandwith or so? i really don't kwow and where i can find an answer for this.
When the blocks are donwloading , sometimes i see : still 6 weeks to go, sometimes 4 weeks. Now i am at about 26 % downloaded . I think it depends a little bit if i'm using my computer for other things or not. It goes faster when i let my computer only download the blocks and i do nothing else on my computer.