Defensively Tulo is a pretty strong upgrade over Reyes, although like Reyes he's a great SS more because of his bat, no so much his glove. From 2013-2015 Reyes is a -7.1UZR/150 compared to Tulo being a +2.7.
This year Tulo is -7.7 compared to Reyes at -6.6, so that could suggest Tulo is in a defensive decline, but using UZR over a half season is usually never a good idea.
If you want to go with more traditional stats, Tulo has 8 errors all season (in more games played), while Reyes has 8 errors in July alone. Reyes has been in a defensive nightmare lately, and while I don't think those awful numbers are sustainable, I do think it's a sign he's regressing to the point of being one of the worst defensive shortstops in the league. Right now Reyes has a .953 fielding percentage - the worst of his career while Tulo is at .977.
In short they are going from a below average / bad fielding shortstop to probably average, but Tulo's bat and power is where they probably get even more value. Reyes can't hit with **** from the right side anymore either, so calling him a switch hitter is a bit generous. It does suck that we are losing a left handed bat with speed though.
In a way this move is like stepping in a time machine going back to 2013 -- Reyes 2.0. It was pretty clear Reyes was regressing, so they take on a very similar shortstop to Reyes (just more power, less speed) in 2013 and hopefully they get more than 2 years out of Tulo before massive regression starts to sink in.