What will change when I replace my wheels from 225/50 R17 with 255/35 R19?

what is the offset of the 19" wheels? You may consider a 19" tire with more sidewall - like a 40 instead of a 35 so your ride comfort won't be harsh. For example, 245/40/19 should suffice.

Other important factors to consider are the weight of each new wheel and tire. You could be adding more unsprung weight on each corner of your car - which can effect acceleration/braking performance.

Just another idea, keep your 17" wheels and install snow tires on them for winter months, while driving on your new 19" set for spring/summer.

I manage tire life on my cars and wife's car by having dedicated sets of winter wheels/tires(I run Continental Viking Contact) as well as dedicated wheels/tires for spring & summer (I run Continental DWS 06 all-seasons). This has allowed me to extend tire life as well as have grip/winter performance living in the Northeast.

Hope this helps.

You may need to tell your insurance company that you've changed the wheels. There would be nothing worse than making a claim and having it rejected because of non-standard wheels.

You mentioned that you've asked the Audi salon about the tyre size. I wouldn't take their word for it. Before jumping the gun and buying/using R19 rims, you may want to check your car's owner manual to confirm that you car can indeed handle R19 rims, and what tyre sizes are recommended for that rim size.

What's more, some countries (at least in Europe) have strict regulations regarding the allowed rim and tyre sizes for a particular vehicle. For instance, in Romania, each vehicle has a state-issued "Vehicle Identity Card", which contains a list of allowed rim and tyre sizes for that particular vehicle.

Using any rim/tyre combinations that are not on the allowed list can (and will!) get you in trouble during the state-mandated roadworthiness tests, and may even be illegal in some places!

Last but not least, as @Ben Thomas mentioned, there may be some insurance-related troubles, too. If you happen to get into a car accident and need to be reimbursed for the damages to your vehicle (assuming that the other driver is at fault, of course), the insurance company may check your vehicle's tyres. If they see that your tyre sizes are not on the approved list, they may refuse to pay for the damages, claiming that you are at fault for the accident because you weren't using approved tyres.