You can change the model, and the changes are updated in SOLR.
However if you are working with a new Model and make some tests, this documents will be indexed with existing properties.
When you modify the model and (for instance) you add a new property, new documents will be indexed with this new property, but old documents are not being indexed with the new one. You can re-index them manually or perform a full re-indexation.
In PROD environments, playing live with the Model is not the recommended approach.