Hi Monika and welcome to the forum.
The functionality of Lookup Maintenenace was enhanced between version 7 and 8 but I don't recall any specific problems as you describe. Check the 'Waht's New' section in the V8 Monarch help for information.)
Out of interest are you starting with a .mod model or a .xmod model in V7?
Do you have Pro or Standard versions of Monarch?
If you have Pro, and also have reasons for changing the lookup table contents quite frequently, it might be easier to use an External lookup and simple maintain a text file somewhere and link to is or distribute it as required.
You should also be able to paste a two column klookup table, held externally, into Monarch's internal table so another option might be to simply amend a master list held externally and completely replace the contents if editing the model using V8.
On very very rare occasions I did see some anomalies which seemed to related to .mod models from older versions which worked BUT may have been imperfect in their internal structure for reasons unknown and which, when interpreted and converted to .xmod files by V8, gave unexpected results because of lines beiong displaced in the structure of the new model. Very rare but could happen. The safest solution was simply to recreate the model.
I hope this is of some help to you.