Will transmission fluid change help slipping?

Its shifting hard, then your problem is electric related ( could be a bad ground wire or transmission sensor or similar ).

It wont hurt to change ATF fluid and filter, then see what happens.  In rare occassions it does fix the problem cause the old filter paper turned into pulp.

I seriously doubt changing the ATF will fix what's worn out. Go on and try the Lucas additive. That's sure cheaper than rebuilding the transmission- and you will need it rebuild some time in the future.

Well, it ain't slipping.That may be the way that transmission is.Best you go to a VW MECHANIC SHOP & ask there because your mechanic don't know.  Explain what it does and see what they say.  VW is not known for building good silky automatic transmissions.

The fluid will not fix it.

Could just be fluid, however your transmission is still in bad condition most likely if the fluid gotthat bad and it was neglected. New fluid isnt gonna change that. So if that does work than your lucky. In extreme cases new fluid can actually make it worse and your car may not move at all. Id try a new filter and fluid and see what happens. No extra additives, just the proper fluid and new OEM filter. Your running on borrowed time regardless.

Likely past due for routine service. Since people almost never read their owners manual, they usually don't pay attention to the transmission until there's a problem, then it can get expensive versus routine maintenance. I don't like things in the tranny that make it thicker or increase viscosity. I do like the Slick 50 brand tranny fluid additive since it doesn't change the viscosity and doesn't have any Teflon in it (at least it didn't used to have Teflon in it going back 30 years) but any decent high mileage tranny fluid like Walmart or Valvoline will work just fine.

I'm not a mechanic but had a similar problem with an old Isuzu.  It shifted HARD intermittently.  (I thought I was leaving my transmission in the street sometimes.)  Mechanics told me to service the transmission with new fluid, etc.  It didn't help at all.  None of them had any good ideas which didn't involve a significant amount of money to just guess at what it might be.  This was years back, but at some point I think there was a conclusion it was probably a sensor that was going bad.  The temperature affected how well it worked.  I sold the vehicle prior to going down that road though.

So in my case, changing the fluid was useless.  

You stated it was slipping in the title. If it's slipping you're going to need a rebuilt transmission soon.

If it just shifts hard I'd do an ATF drain and fill.

DON'T flush it.

A fluid charge rarely fixes a slipping or shifting problem and it can even make it worse. In other words, the damage has already been done. You can look at it two ways; cheap to try - if it works you got off easy. Or, waste of money since you'll probably need a trans rebuild anyway.

No idea.  Is your mechanic a goood one?  If so it might help.

Or it could be a problem with the transmission cooler. 

I'd go for a second opinion and then decide what to do.