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? I have installed v0.21 this working and it is working but not as v0.20.1. The version 0.20.1 downloaded constantly. Now it stops from time to time during a few minutes and than it continues again.
now still 234406 blocks to go. 3 weeks to go comes on my screen.
A am gonna answer on the other posts soon, but i am first gonna the try to find the papers of my computer i bought about 4 years ago. Have to search a little bit.