Multiple document schema in same collection mapping to POJO

I have multiple excel templates with versions. Between every template version the structure of data varies i.e. columns/rows, different pages… etc.
For each excel template version I’ve created its own document schema. I want to store all the documents in the same collection regardless of the excel template version (as there are too many versions for multiple collections).
Question is, how do I map different schemas into a single POJO. As the structure changes between each template version I’m ending up creating a unique POJO class for each schema!!
If I need to create a POJO for each excel template version, and my application code needs to change every time a schema change is introduced (like addition of a key/field) what is the advantage of using mongoDB vs RDBMS in my case? mongo promises no fixed schemas but practically I need to separate schemas to new collections due to POJO mapping?? Are there any dynamic mapping strategies I’ve missed to avoid application update every time a new schema is added to the collection?

Thanks in advance.