Need to re-index after upgrading solr

[LibrErli]

hi, i used your tool to upgrade an index from solr 3.4 to 6.6. It works pretty well, but running queries on the index occurs some errors. A query about all documents *:* returns the expected result, but if i want to query with a specific field, solr returns that the field is undefined. So it is necessary to re-index the index.
Do you know this issue? Do you always re-index the index after upgrading it?

[janhoy]

You can only upgrade an index if the schemas are compatible and no FieldTypes have been removed or changed in between. Great care has to be taken in order to research that your schema will be compatible, and if you see that there is a deprecation/removal or other breaking change, you should first update the current index accordingly before making the switch.

So in your case, are you sure that you e.g. use TrieIntField both in v3.4 and 6.6? Can you be more specific?