The basic 3.5–Pathfinder conversion rules cover the swordsage class itself. None of the class features of the swordsage, aside from the maneuvers, need special care in Pathfinder. The basic conversion guide covers class skills, CMB/CMD, and so on.
The maneuvers themselves are fine in Pathfinder, mostly.1 Only the Diamond Mind discipline, with its emphasis on Concentration, is a problem.
Handling Concentration basically has three options:
Use spellcaster-like concentration checks, rolling d20 + initiator level + ability score. This has a few sub-choices:
Which ability? Wisdom would be the closest to the spellcaster style for a swordsage, but Constitution would be closer to the functioning of 3.5 Concentration.
A skill would have a +3 bonus over what this check does. Should the swordsage get a +3 bonus for this?
Create a new, Constitution-based, skill that’s basically just for Diamond Mind and the feats in Tome of Battle. On the one hand, that’s a really niche skill. On the other, unless you were going for a swordsage gish, this is pretty much the same story as the 3.5 swordsage had—it’s not like the 3.5 Concentration skill was ever used for anything else anyway. Call it “Focus” or something. This new skill is a class skill for all three initiating base classes.
Use an existing skill—Sense Motive is a common choice. Optionally swapping the skill’s ability score for Constitution when used with Diamond Mind. Whatever skill you choose should be considered a class skill for all three initiating base classes.
Which you choose kind of depends what you’re aiming to get.
The first is most consistent with Pathfinder as a whole; personally, I would favor basing it on Constitution (after all, Diamond Mind is also available to warblades), and I would put in the +3 bonus—Tome of Battle is well-balanced, and balanced around that. This approach does save the Diamond Mind user a skill point per level compared to 3.5, and prevents other classes from potentially contributing as much (only half-level, when in 3.5 they may have had Concentration in-class and been able to contribute full-level), which are concerns.
The second is probably most consistent with Tome of Battle itself, but is very awkward with multiclass initiator/spellcasters, and really, the new skill is super niche. That is awkward.
The third is probably the most popular in my experience, but it also has issues. If you don’t change the ability score to Constitution, you end up with Diamond Mind losing potency for warblades, and potentially becoming even better for swordsages. It also is at least a little awkward for multiclass initiator/spellcasters.
My personal recommendation is to use a Constitution-based Sense Motive check for Diamond Mind Concentration checks, and for Concentration as used in Tome of Battle feats. This seems to me to cover the most bases and fit the game the best.
- One thing I have gathered from discussions with the Path of War development team (I freelance for Dreamscarred Press) is that Tome of Battle and/or Path of War slot into Pathfinder somewhat more awkwardly than in 3.5. The reason for this is because 3.5, over the course of many books, started to try to fix the various mistakes made in core, to tighten the gap between mundane and magical characters. This effort culminated with Tome of Battle, but numerous other entries, particularly Complete Champion, also helped the situation. As a result, it is definitely possible to produce a 3.5 non-Tome of Battle mundane character on par with an initiator. It was easier to do with an initiator, but the option was there. Pathfinder, by contrast, has kept very closely to the balance of core—which would be an admirable resistance to power creep if core weren’t so badly balanced. There is a dearth of good, solid feat choices for many types of martial character in Pathfinder. You may find that, unless you either import other 3.5 material alongside Tome of Battle to give other martial characters better feat options, it will be very hard for non-initiator martials to feel that they contribute evenly with your swordsage. However, I strongly feel that the problem here is with Pathfinder—not with Tome of Battle. Playing “3.PF” works much better in my experience.