on the upgrade documentation page http://www.mailpiler.org/wiki/current:upgrade
in chapter “from 1.3.0 to 1.3.5“ you describe step by step how to keep the utf8 charset but not how migrate from utf8 to utf8mb4.
But since new piler installations use the utf8mb4 this would be the preferred path to take for an upgrade also. But for this procedure you just refer to an Bitbucket issue from 2016. For me it is not clear from this issue which steps need to be taken.
Please provide step by step instructions on how to migrate the piler database from utf8 to utf8mb4.