In case you missed it last night, the Buffalo Bills are changing defensive schemes again: Jim Schwartz will be departing his defensive coordinator post, to be replaced by Dennis Thurman, who will run the defense that new head coach Rex Ryan has run for the past decade.

It will not be a radical shift for the Bills, who ran this defense during the 2013 season under Mike Pettine, a Ryan disciple. But any time we talk about system shifts - and this, for the record, will be the Bills' fifth system shift in as many seasons on defense - there are always philosophical and personnel tweaks to consider. Let's do that this morning.

Base defense

Many fans will complain about switching from a 4-3 back to a 3-4, but those that do aren't grasping what Ryan's defense really is. First and foremost, keep in mind that a base defense is used less than 50 percent of the time in today's pass-happy NFL, so these tweaks are only going to matter on 40-45 percent of plays next season.

Ryan's defense is actually a 4-3 Under. It looks like a 3-4, but it plays like... well, however Ryan wants it to play like on a given down. The hallmark of the scheme is disguise, and versatility is a necessary asset for specific members of the front seven, specifically the strong-side linebacker.

Here's what the front will look like under Ryan:

Particularly in the lower diagram, you see how the scheme remains a 4-3, despite the appearance of a 3-4: one of the outside linebackers (the weak-side player), in Buffalo's case Mario Williams, plays with a hand in the dirt like an end. The other outside linebacker has more of a contain-coverage role on base downs; Jerry Hughes is capable of playing it, and so is Manny Lawson.

The biggest difference is that the front takes a traditional linebacker off the field in favor of an extra defensive lineman. The Bills will likely be in the market for a five-technique end to play next to Marcell Dareus and Kyle Williams up front (they could also open that spot up to a competition among a trio of reserves), and barring any sort of personnel move, there is going to be a ferocious battle for two linebacker jobs between Kiko Alonso, Nigel Bradham, and Preston Brown.

Pass rush preferences

One of the biggest points of contention last offseason, when the Bills were switching from Pettine's scheme to Schwartz's, was that the Bills were ultimately going to do much less creative blitzing in 2014. Those concerns went by the wayside when the Bills still led the league in sacks and finished with the third-ranked pass defense in the league.

Now, there are concerns that the Bills are going back to the blitz-happy ways that fans were desperate to see retained this time last year. Ryan has the luxury of relying on a four-man rush in Buffalo if he wants to - especially if the team is able to retain Hughes, an upcoming free agent - but Ryan being Ryan, we're going to see a return to the overload blitzing that we saw under Pettine in 2013. That's just how Ryan prefers to play it.

Formations and personnel packages

Another key difference between the two systems is the fact that Ryan's defense will use a far greater variety of formations and personnel packages than Schwartz's did. Where Schwartz largely switched between his base 4-3 and a nickel defense in passing situations, with very little dime mixed in, Ryan will use more dime than nearly every other team in the league, mix in some quarters, and still use a ton of nickel, as well - 2-4-5, 3-4-5, 4-3-5, or otherwise. Again, disguising intent is the name of the game, and this is one way to achieve that end.

Coverage preferences

Combine the changing personnel packages with Ryan's blitz-heavy tendencies, and you stumble across another key difference: the way coverages are structured.

The Bills emerged as one of the league's two or three best pass defenses under Schwartz because, aided by the league's best four-man pass rush, Schwartz was able to mix up coverages, switching between man, zone, and man under on a whim, leaving zero safeties high on some plays and three on others. The team's surge in quality linebacker play was a big reason for their success defending the pass, as well.

Under Ryan, the Bills will look more like they did in 2013 - there will be a heavier emphasis on man coverage given the blitz tendencies. So while there will still be coverage variation and disguise under Ryan, there will still be a slight shift in philosophy on the back end.

Buffalo's secondary made a seamless transition between 2013-14 thanks to the great coaching efforts of defensive backs coach Donnie Henderson. As Ryan poaches most of his former Jets defensive assistants for his coaching staff in Buffalo, it would be lovely if he'd consider keeping Buffalo's best position coach around. Working in favor of that goal: Ryan and Henderson both served as position coaches in Baltimore from 2000-03, with Henderson coaching the secondary under Brian Billick, and Ryan coaching the defensive line.

Bottom line: there is enough that is philosophically different about Ryan's defense, compared to Schwartz's, that would have made a marriage of the two virtually impossible. But while another scheme change might be frustrating to some fans, the Bills are going back in a direction that has very recently worked for them - and they have the league's best defensive mind helping them make the transition.