By default, if users run simply 'ant update' to upgrade a DSpace instance, the Solr Configurations & Schemas will NOT upgrade properly. (Note: However, if you run 'ant -Doverwrite=true update', then Solr will upgrade properly)
The reason, is that DSpace's Solr directory updates it configurations similar to the [dspace]/config/ directory.
So, if you simply run 'ant update', all your existing, older Solr Schemas & Configurations will remain in place unchanged (and new configs/schemas will be suffixed with ".new", e.g. schema.xml.new). This could cause instability in DSpace if users are accidentally using an old Solr schemas/configurations with a new DSpace API.
I think we need to always overwrite existing Solr Schemas & Configurations. If we want to keep around a copy of older Solr Schemas, they should be suffixed with "-[date].old" (like when running 'ant -Doverwrite=true update'). As users are less likely to customize Solr Schemas/Configs (without knowing what they are doing), I think it's better to default to overwriting these files during an upgrade.
As it stands, most users will not even realize there are Solr configurations under [dspace]/solr/search/conf/ and [dspace]/solr/statistics/conf/ which may not have upgraded properly.