Hi,
Diazepam has a longer half life than Lorazepam and therefore remains in your body a lot longer which makes it easier on your body during withdrawal.
So your Dr.will probably first cross you over to Diazepam at an equivalent dose to the Lorazapam you are taking now so that you are still getting the same amount of the drug before he/she starts to taper your dosage down.I think 3 mg Lorazepam is equivalent to 30mg Diazepam.
When you are settled on the Diazepam your Doc. will probably agree a taper program with you, this is usually done by cuts of 10% of the previous dose every two weeks or so until you are down to about 2mg then he/she will probably prescribe you liquid Diazepam to complete the withdrawal, it's easier to use liquid than try to cut a 2mg tablet into small bits.
There's some info here that might be helpful to you,
http://www.benzo.org.uk/manual/index.htm
Remember you are in control of this and don't let your Dr. rush you, take as long as you are comfortable with.
Hope this helps
Decca.