There have been an number of recent threads on this topic.

In my opinion, your transmission is already shot and changing the fluid now will not help it, and will likely "reveal" the pre-exisiting problem soon after the flush.

Changing fluid on a regular basis will not cause the tranmission to slip but changing fluid on a high-mileage tranmission that has not previously been flushed and which is already showing signs of failure, appears to "cause" the transmission to slip. I put the word cause in quotes because the flush doesn't cause the failure, it simply reveals the failure.

I've talked to several automatic transmission engineers and have gotten two rationals for this failure scenario.

First scenario says that the friction surfaces in your transmission are already glazed and slipping, but that slipping is being offset by the decrease in lubrication of the fluid. So, your transmission still works but it is likely operating at an elevated temperature and probably does not have the ultimate power transmission capability of a regularly maintained unit.

The second scenario says that the seals in your tranmsission are protected by plasticizers in fresh tranmsission fluid and without fresh fluid, your seals become brittle and weakened. When you put new fluid in, with greater viscosity and, hence, greater pressure, these seals quickly fail. Tranmsissions which have their fluid changed regularly have seals which are regularly bathed in fresh plasticizers and those seals remain more flexible.

I'm not sure of the validity of either of these explanations except to say that there appears to be a tie between transmission failures and high-mileage flushes for cars which have not previously had flushes.

I would save your money.

Ken