L322 Mileage Reading

Ive been having a bit of a mare on an engine exchange on my L322, a replacement engine (which has to come back out - very long story) had been fitted but when I went to check the progress the mileage display had gone from 140k to 999999.
Garage said it wasnt a problem and that it can be reset to the correct value but is this normal?
Is there any simply way of clearing is back to what it should be?

Yeah this is pretty normal if ECUs or things have been changed. Basically its to prevent dishonest folk buying low mileage ECUs and fitting them to high mileage cars. Basically the car either adds the old mileage to the current one or just puts it sky high like 999999.

The Mileage is stored in the Instrument pack and also the LCM....both must be identical else you will get the 'Tamper' indicator showing (small dot next to the reading) and will default to the higher reading.

I am not sure if the reading can be changed, perhaps with dealer level diagnostics maybe....

Blimey, I hope it can be changed as a newly rebuilt engine is going in next week and the last thing I want is for it to show monster miles. I dont want it to show anything untoward, just the previous mileage.
Sounds like its going to have to be a main dealer I guess for this or an inde with all the gear.