I'm currently running dev version 1.83/9276 and the release notes ask us to reindex everything:
Older indexes are migrated from 4_10 to 5_2. However, the new index
structure is more efficient and we recommend to re-index everything.
Please use the index export before you do the update to a large
surrogate xml file. After the update, start with an empty index and then
initialize this with your dump.
I'm not confident to do that.
Can someone be more specific on that procedure?