“Trail” is the distance that the front wheel axle trails (geddit?) behind the wheel’s contact patch.
The trail figure is dependent on a few things – head angle, fork offset, and wheel size.
A smaller trail figure gives faster handling.
Steepening the head angle reduces trail. Slackening HA increases trail.
Increasing offset reduces trail. Decreasing offset increases it.
Increasing offset rather than steepening HA gives responsive handling, without the “riding the front wheel” sensation that a steep HA gives, as the front wheel is further in front.
Since the advent of suspension forks, the offset figure has been largely standardised (with notable exceptions like Bontrager), so trail adjustments have been made by HA adjustments.
The bigger wheels of 29’ers also increases trail (hence early examples with 72 deg HAs), so increased offset on big wheels has a more noticeable benefit.
If you’ve got 29’er 34’s (which have 51mm offset) stick with that set-up unless you want to slow down the steering response. Only thing to add is that axle-crown lengths may differ between the forks too which would affect HA and therefore trail too.