You're correct that SEE 11.1.2 should not be used when attempting to upgrade to Win10 1903, as it is not supported for that target platform. Some items were added to later versions of SEE to make the 7 to 10 migration much easier as well. Your first step would be up get that SEE agent upgraded.
If your company is allowing the upgrade to 1903, I would imagine they would have tested that with Disk Encryption in your environment including those with Disk Encryption on their machines. Hopefully that is the case. Perhaps they have an upgraded server but your agent is just not up to date. Are you able to request and upgrade to your SEE agent through your IT team? I would also ask them if they have prepared for OS migrations like yours for that upgrade package.
Your other option would be to decrypt your hard drive completely and then update to Win10, but even then the SEE 11.1.2 agent is still incompatible with 1903. For 1903, you would need at least SEE 11.3.0, and the most recent version is 11.3.1.