You need to make sure that LastPass first has all those passwords in its database before you delete them, but once that's done, yes, delete them all from your browser and LastPass will log you in whenever you need.
I also used an encrypted text file for all my passwords in the past, that way I could add as many notes as I needed about any entry and just keep it in alpha order. I encrypt it with
AxCrypt, free encryption software that works very well. I still keep that encrypted file just in case, but it was always easier than a spreadsheet that might need a program to open. My text file just opens in Notepad.