L1B extension post I140 approval

[PRN]

Hi, I am on a new L1B with validity until the end of 2018 (with a possible extension for another 2 years).  My employer is filing for a GC. Say, the I140 is approved the first or second quarter of 2018 with L1B still valid until end of the year, what will happen to the visa status

1. Should H1B be filed immediately as the I140 is now approved?
2. Continue on L1B until the end of the year and the only option is to apply for an L1B extension (and not H1B) to stay beyond 2018 since the L1B period( of 5 years) is not maxed out yet? 

Please note that I was on H1B earlier and stayed in the US for the entire 6 yeares on H1B. Returned back to the US on L1B. 

Please advice. Thank You