Aerials are different, your conventional aerial is banded (set to receive the part of the band which your local transmitter uses) Digital transmissions come from the same transmitter but use all or a different part of the tuning band that your conventional aerial cannot see very well, the result of this is usually missing channels or weak reception. Weak reception, because digital is sent as data, interference or weak signal levels show as picture breakup (little patterns of squares on the picture, may come and go)
Upgrades are a myth, when you change to digital you will need a new aerial, (wide band high gain) in this area you will probably need a masthead amplifier, and double screened cable, this leaves the mast, bracket and fixings which are the cheaper bits and it makes sense to replace any ageing or failing ones at this time, so you have a complete aerial system not an upgrade, certain areas may not need a masthead amplifier but it's still worth replacing the cable and fittings as it's more expensive to get someone back at a later date.
Sky is digital, it's a different method of transmission as it comes via a satellite, through your dish to your sky receiver and then to your TV, the problem with sky is you can only watch one channel at any time in all rooms connected, you can get multiroom which means another receiver in another room but as everything extra with sky this costs monthly, (£120 per year extra) with freeview digital you can put a freeview receiver in any connected room and watch anything anywhere. Freeview is not set up to compete with sky, it's to replace the existing analogue channels.
