EA really mucked this one up. You can use a cheat to view the lot class value, and another cheat to clear the override flag (which will remove the fixed lot class value and allow the game to recalculate the lot class value), but setting a value will not stick because the cheat to set a value only works once the override flag is clear. There is no cheat to set the override flag.
You can set a permanent lot class value using the LotAdjuster. Check the Lot Class checkbox and type in the permanent Lot Class Value that you want. A specific lot class value can be high-, middle-, or low-class depending upon the type of lot, so you might have to experiment a bit.
If you don't like to use the LotAdjuster, you can also use SimPE to change the hex values. The wiki contains information about the lot class fields:
http://www.sims2wiki.info/wiki.php?title=DESCThe values not sticking could have something to do with the cost of the lot.
(See Mootilda's post here
http://nene.modthesims2.com/showthread.php?p=2048000#post2048000)
Also in that thread, there are suggestions that neighbouring lots may have some influence. So maybe surrounding the slum with abandonated cars and dumsters would help.
Abandoned cars and dumpsters have no affect on the lot class value; in fact, I was unable to find any effect from decorations during my research. Only
lots which are within 6 tiles of your lot will have an effect. However, the effect can be bizarre because of the difference in lot class ranges for the different kinds of lots. For example, apartments need a higher lot class value than residential lots for each lot class. This means that a house set among cheap low-class apartments will probably be considered high-class. As well, the EA lot class logic doesn't seem to work for community lots, so having parks around your house can actually make an expensive high-class house be considered low-class by the game. Sigh.
The link specified above occurs fairly late in the discussion... a better place to start reading might be:
http://www.modthesims2.com/showthread.php?p=2025181#post2025181