Maintenance plan grants access to three previous versions of software. However, it doesn’t grant additional licenses, so when using single-user license only one version of software can be used simultaneously. When multi-user licensing is used each version consumes one license. With single-user license, Contract Manager requests new serial number for activation according to these instructions .
Note that if previous version has been acquired earlier and license has been under maintenance plan uninterruptedly, can existing serial number be used during activation. With multi-user licensing, access to previous version is automatically included in lic-file. More information is available here