Many thanks for clarifying that the answer is yes. I feared I would have to create a new email address for the IMAP account, and then go through move messages, delete old account and address, recreate old address and account as IMAP, move messages, delete new account and address.
I’m accepting it as the solution to the question I asked, but for the benefit of anyone else thinking that changing to IMAP is the easy way to see what is being blocked by the email server, and who wants to store their emails locally and not on the server, I add the following subsequent experiences.
Creating the IMAP account in eM Client, I came up against the limit of 2 accounts in the free version. I already had two accounts: my POP3 account, and an account for a different email address required for verification by my bank which doesn’t accept my normal address for some reason. I deleted the bank account., to be recreated later, and created the IMAP account in its place.
Now I tried to “move” the folders from the POP3 account to the IMAP account. I got “Special folder cannot be moved”. I eventually discovered that the folders should not be “moved”, but use right-click and “Copy Folder”, which is better but should not be described as “moving”.
After copying the folders I discovered that they had also all been synched to webmail, which is the very thing I most want to avoid. (I’m considering IMAP only as a means to see in eM Client the webmail spam folder.) So forget about IMAP, delete everthing that has just been sent to webmail, delete the IMAP account in eM Client, recreate the “bank” account, and we’re back to where we started.
The solution was MailWasher which I have always used as a pre-filter to eM Client (and to Outlook Express before that). Although basically a spam blocker, it serves my purpose of showing what alleged spam is being blocked by the email server. I had to upgrade my version, and then all I had to do was change the description of my account from POP3 to IMAP by ticking a box. (Possible because MailWasher does not store messages, only displays them, and can delete them or bounce them or leave them to pass to the email client.) Crucially, in IMAP mode, it can display from the server’s spam folder, as well as from the server’s inbox.