Make a newer version of the LDF-file work with an older version of the MDF-file
Is it possible to combine a "newer version" of the LDF file with an older version of the MDF file and keep changes logged in the LDF?


Assuming you want the database to be transactionally consistent, no.


  Just attaching the MDF and LDF gives an error 


In this case it's because the log number is no valid, for whatever reason given the database file's metadata. This is a symptom of the exact process you've described. In this case it'd just be better to use the create database to regenerate the log (or DBCC commands) because either way the database isn't going to be consistent which it already wouldn't be with the above steps you've outlined. Six of one, half dozen the other.