IF I buy SolidWorks 2021 license from vendor , Can I install SolidWorks 2018 version and after working couple of months, can I move to 2021 again without any extra charge ?
Yes it will be no problem. Strictly speaking if you can get the download you can go as far back as you want to.
With regards to 2021, personally I would stick to 2020 sp5 until at least 2021 sp3. You will not see a big difference between the functionality of 2020 and 2021 so there is no rush.
Should be free to switch between versions.
Download all the installation files first.
Do you have a license for 2018?
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