F1 to H1B - Should I be filing taxes for 2019 as a Resident Alien or a Non-resident alien?

Your days on F1 in 2019 were exempt from the Substantial Presence Test (since you had only been an exempt individual for some part of 4 previous calendar years). Your days in H1b status in 2019 were enough to pass the Substantial Presence Test for 2019, so you were a resident alien for 2019. However, since you were a nonresident alien in previous years, the First Year of Residency rules apply, which means your residency starting date was the first date you were present in the US for the purposes of the Substantial Presence Test in 2019 (i.e. the date you changed into H1b status). So you were a dual-status alien for 2019: resident alien since 6/5/2019, and nonresident alien before then.