If your old aerial still works well then there's no immediate need to change it. The main difference between the two types of aerial is that the newer ones are 75 Ohm balanced. Although the digital transmission system is pretty robust, it does have a problem that it is more susceptible to picking up noise than the older analogue TV system. This is why cable for digital TV reception has two lots of shielding compared to older generation coax with just one.
When it comes to the aerials, those originally installed for analogue only didn't have any mechanism to help block noise because it wasn't a problem. The newer aerials used for Freeview reception have something called a balun. This acts in a controlled way to break the direct connection between the cable insulation and the aerial. As a result, problems with noise are greatly reduced.
We spend a lt of time looking at- and minimising the effects of noise in aerial installations for digital TV transmissions. Noise affects signal quality. With digital, signal quality is far more important than signal power. It's always possible to increase signal power with some form of amplification. But signal quality can never be improved. It's at its best direct off the back of the aerial connection. Everything else we do either preserves it or reduces it, so we try hard to preserve it and avoid reducing it wherever possible.
The honest answer to your question of "How much....?" is there's no definitive answer. There are just too many variables to be that specific.
Personally, I would replace the aerial for the simple reason that doing it as part of one whole job is cheaper than coming back a second time. The type of aerial you need depends on where you live and also what's happening with TV transmission frequencies from your local transmitter over the next 10 years. The Government is determined to sell off chunks of the TV transmission bands to the mobile phone operators. This means finding an aerial that works well now but will also work well in the future when the TV frequencies have all been shifted.
In my part of the country (N.West England) that means choosing a Log Periodic aerial. For other parts of the country it could mean a Narrow Band Group aerial. There's also the question of whether the aerial needs amplification. This could simply be because the house is on the fringe area of the local transmitter, or you might have trees or a hill in direct line of sight between your property and the transmitter.
The answers to some of these questions come from local knowledge. The rest of the time though it needs direct measurement with a good (read 'expensive') aerial meter to see what the local conditions are like.
Once the aerial is sorted (plus any bracket work if bolts and nuts are rusted to hell), then it's diagnosing what's happening with the amplification in the house. This could be a 10 minute job "Oh, there's no masthead/loft amp power supply or it has failed". Or one could end up inspecting all the cable from the roof to the amp for breaks (more likely, just replacing it), or diagnosing a fault in the supply wiring or a failed masthead/loft amp.
