SQLite Forum

Setting non-binary columns to null in a table with a binary column populated with data creates free space and a larger file
That appears to be true, however, if you put the blob at the "end" of the record the contents of the overflow pages are still overwritten when the payload length changes but are not overwritten if the payload length stays the same.

So yes, putting "long fields" (or just long records) that have overflow lists means that the entire overflow must be read through and decoded in order to get to the data which appears "after", however, when you re-write a record and the size of the payload changes, the entire cell (including the overflow pages) are all re-written.