To avoid the stupid problem of the DropBox icons overwriting the OneDrive and OneDrive for Business icons explained in OneDrive: sync icons not showing problem, I've chosen to uninstall DropBox.
In the end, I was using it only to share my 1Password vaults between PC, Mac and iPhone/iPad. OneDrive has the advantage of being integrated in Windows and also with Office 365 you have 1Tb of space for free.
On the Mac:
- go to 1Password preferences, sync tab, and change it to folder (choose a folder inside OneDrive)
- it will ask if you are sure to delete the files in DropBox, take risk and say yes
- to uninstall DropBox: close it (from the top right menu), and from the Activity Monitor kill all the DropBox processes still running. Then you can drag the application icon to the trask
- in Finder, search for DropBox, you will still find a folder in your profile; delete it
On the PC:
- run both the DropBox and OneDrive sync
- open 1Password, it will automatically ask to open the vault from another source; choose the OneDrive folder
- uninstall DropBox
- search for all the DropBox folders and delete them (this can require the reboot of the computer)
On the iPhone/iPad:
- so far you can't use directly the OneDrive folder, so you need to use wifi sync
- from the Mac, Preferences | Wifi Server, check "Run a Wi-Fi server trom this Mac"
- from the iPhone/iPad, start 1Password and dismiss the message about DropBox
- from Settings | Sync | Start Syncing | Sync using Wi-Fi, select your Mac and enter the secret phrase from the Mac, then finally you can "Sync Now"