The slightly longer answer is to be found in (EC) 3821/85, but basically you do not have to replace defective analogue Recording Equipment with digital unless;
The vehicle was first used on or after 01/01/1996
The premitted weight exceeds 12 tonnes
The Recording Equipment only uses electrical impulses
and the Recording Equipment breaks down.
The Definition of Recording Equipment is ‘the total equipment used to show and record driving and other periods of activity of a vehicle and its’ crew’ Total Equipment = the tacho head, the motion sensor and the connecting cable. It is very unlikely that Recording Equipment would fail simultaneously. Therefore the chances of having to replace analogue with digital in your case (with a 7.5 tonne vehicle) are ZERO.