There are collation changes between utf8 and utf8mb4. Specifically, utf8mb4 uses the (much better) UCA 9.0 algorithm.
But this implies that all indexes will have to be regenerated, if they contain a column that is utf8 and will be converted. As they sort differently, this requires rebuilding the index from scratch. Depending on data size this can take measurable time and effort.
But this implies that all indexes will have to be regenerated, if they contain a column that is utf8 and will be converted. As they sort differently, this requires rebuilding the index from scratch. Depending on data size this can take measurable time and effort.