Upgrade to AltaMail. You can import all of your content (settings, emails and files) from AltaMail Classic or AltaMail Go. When you open AltaMail for the first time, you will see a new account screen. If you have AltaMail Classic or AltaMail Go installed on the same device, you should see "Restore from" options at the top of the new account screen. Tap on the "Restore from..." button to import the settings.
The 'Full import' option will import all settings and email content if you have the available storage space on your device to save the import file.
If you have problems with space, then the 'Quick Import' option will import all settings except for the message content of emails. The email summary will be imported, but not the content (message/attachments). Email content will then be downloaded from your email server when you view a folder.

You can also import your settings manually. Open the AltaMail Classic (or Go) app, open the app settings and scroll down to the bottom to the Application section - Backup/Restore. Do a backup of the old app and save the file in iCloud or other location. You can save the backup file to your Mac or PC with the free WePrint (View)
Open AltaMail and open the same Backup/Restore screen. Tap on the Restore option and choose the location of your backup file.

If you have get errors with storage space on your device, you can reduce the backup file size in the backup screen when you do a backup. Tap on Settings then turn off "Include email content" so emails will be downloaded again in AltaMail.

You may also want to do the restore in two stages if you get cloud server import errors. First open the "Files" screen in the left side panel. Then tap on the "+" button to import the backup file from iCloud or another location. This will store the file locally for easier access. Then you can open the app settings and view the Application section. Open the Backup/Restore screen and restore from the the saved backup file in the local Files screen.

Later after the first install, you can also show the new account screen to import settings in the left side menu - and tap on the "+" button beside the Accounts button. Note. using the "Restore from..." option will clear all settings from AltaMail and replace them with the imported app settings.

If you have enabled "Synchronize settings" in AltaMail Classic or AltaMail Go then you may see a button "Email Accounts" in the top of the new account screen inside AltaMail. This will allow you to import just the account settings from the other app.