No, not all 4am stores are switching to 6am.
Yes, many (possibly all?) areas are restructuring their shift differential.
You will earn shift differential for any hours worked between 10pm and 8am (possibly 11pm for the stores that close at 11pm? I'm not 100% clear on that, but it's definitely 10pm at my 10pm-close store), but only if you work a minimum of 3 hours during that period in a shift. Any hours outside that window will not earn differential, period.
The raise to base pay is based on a formula that looks at all the hours you worked in the past year and calculates how many of those hours DID earn shift differential under the old system but would NOT earn shift differential under the new system. If you worked 4am-12:30pm every day and always took your lunch after 8am (assuming a 30 minute lunch state), that would put exactly half of your hours in the new window and half of them outside of it, so you would receive a base pay increase of exactly half of your local shift differential (here, it's a dollar). Of course, if you weren't working shift-differential shifts all of the time, you'll get less; for instance, I only moved to a 4am workcenter halfway through the period they're looking at, plus I've had to work later shifts on the weekends to make up for gaps in our TL rotation, so I only ended up getting 18 cents. On the other hand, we have a TM who worked 5am-1:30pm almost every day for the past year, which means MORE than half of his hours earned differential before but won't now; he got something like 62 cents.
Important note--hours under the Remodel workcenter are NOT included in the calculation in any way, which sucks for me since I spent 8 weeks working at 4am to help another store through their remodel after running my own. Oh well.
The upside: the base pay increase also applies to paid vacation/leave time if you have/take any; it stays with you if you change workcenters in the future; and it'll make your future merit increases a bit bigger than they would have been before. Short term, if you work the same hours you did last year, you should be just about breaking even. Long term, you should benefit (if only slightly).