Wanted to add my experience with converting. I had many text fields in ancient Linq2SQL code. This was to allow text columns present in indexes to be rebuilt ONLINE.
First I've known about the benefits for years, but always assumed that converting would mean some scary long queries where SQL Server would have to rebuild the table and copy everything over, bringing down my websites and raising my heartrate.
I was also concerned that the Linq2SQL could cause errors if it was doing some kind of verification of the column type.
Happy to report though, that the ALTER commands returned INSTANTLY - so they are definitely only changing table metadata. There may be some offline work happening to bring <8000 character data back to be in-table, but the ALTER command was instant.
I ran the following to find all columns needing conversion:
SELECT concat('ALTER TABLE dbo.[', table_name, '] ALTER COLUMN [', column_name, '] VARCHAR(MAX)'), table_name, column_name
FROM information_schema.columns where data_type = 'TEXT' order by table_name, column_name
SELECT concat('ALTER TABLE dbo.[', table_name, '] ALTER COLUMN [', column_name, '] NVARCHAR(MAX)'), table_name, column_name
FROM information_schema.columns where data_type = 'NTEXT' order by table_name, column_name
This gave me a nice list of queries, which I just selected and copied to a new window. Like I said - running this was instant.
Linq2SQL is pretty ancient - it uses a designer that you drag tables onto. The situation may be more complex for EF Code first but I haven't tackled that yet.