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?