I have QB-desktop 2018 and would like to try QB-desktop-2020

Yes you can. Make sure creating a backup file and restore it with another name to be open on your trial license of QBD 2020.

Thanks so much for your answer.

I'd like to be certain: if I do 30 days of work with qb2020, do a backup, then restore it in qb2018 I will retain all the work I did in qb2020 for those 30 days. Right?

Once a company file is converted on QBD 2020, you are unable to open it on QBD 2019 or older.

Thanks for your response, but I don't understand- that contradicts what you said earlier.

Please give me the detailed steps for me to switch from qbd2018 to qbd2020 for 30 days, then switch back to qb2018 and retain the work I did for those 30 days.

Thanks!

As I mentioned earlier, create a backup file and restore it with another name. You will have 2 different company files with the same data (i.e File A.QBW, Fie B.QBW). Open the File B.QBW on the trial license QBD 2020 for your evaluation. You may run both QB versions at the same time. You can open File A.QBW on QBD 2018 and File B.QBW on the trial license of QBD 2020. 

I'm not worried about opening my qb2018 files in qb2020. I might want to go the other way after I try qb2020, but don't want to lose any of the work I did in qb2020.

Can qb2018 open the file I had open in qb2020? Or, if I make a backup in qb2020 can I restore them in qb2018?

Thanks!

I already have answered your question. Once a company file is converted on QBD 2020, you are unable to open it on QBD 2019 or older. You will need a 3rd party tool to migrate data.