Those who can
The entire secondary is by and large unaffected by a potential change to a 3-4. So let’s just assume that all four of the starters could play both, just to simply this conversation. Firstly, the duo of Joe Schobert and Christian Kirksey could absolutely adapt to the 3-4 scheme. Schobert and Kirksey are smaller compared to conventional middle linebackers in the NFL. Most of the time NLF middle linebackers could topple 250 pounds in the field. Both Kirksey and Schobert are far below that threshold and can be considered “small” (weight wise) for middle linebackers.
In a 3-4, you need more athletic linebackers and both Kirksey and Schobert fit that bill. They both have great sideline to sideline quickness, meaning they can still be efficient in passing downs and coverage. They’re also the best tacklers on the team, making their ability to play in the middle a mandated necessity. You can’t have big, lumbering players in this scheme that have poor fundamentals.
As for the star of the front seven, Myles Garrett, he could arguably play either end or outside linebacker. Garrett is nearly 300 pounds and considering a 3-4 defensive end needs to be heavy like a traditional 4-3 tackle, Garrett has the size and strength to play the position; much like J.J. Watt does in Houston. Watt can get his sacks in a 3-4, and arguably so could Garrett. Garett’s also fast enough to play outside linebacker as well, meaning he could probably do great work as a stand-up player.
Garrett has never shown on a pro-level that he can play in the underneath on defense in pass coverage. Nor has he shown he can play the run defense from off the line of scrimmage. He probably can, so it’s not likely that he’d be a liability. He’s athletic enough to adjust, he would just need proper coaching.