Will it be harder to get into university?

Not at all. What you actually need to achieve to get on to a particular course will remain the same; how this is displayed will change.

Will universities count both your AS and A-levels?

No, universities will only count your highest level of achievement. So if you take a subject at both AS and A-level, they will only count the A-level.

Will all universities use the new Tariff?

No. As it stands, only a third of undergraduate courses list a Tariff entry requirement anyway; the rest just list grades. Ucas expects this to remain pretty much the same. Under the new Tariff, AS levels will be worth 40% of an overall A-level as opposed to 50%.