In your first question you say the part was added from the 'local' library, but this needs some clarification before we can provide a solution. Are you using the Expedition flow with a Central Library or the netlist flow with libraries stored in the file structure?
If Expedition with a Central Library then are you using 'Edit local symbol' to update the symbol? If you are then the update should be immediate - you could try running Reset Text Properties after making the changes to see if this removes the old properties.
If the symbol comes from the Central Library then ensure you run Tools - Update Libraries before using Tools - Update Symbols, if the symbol has changed (new timestamp) then it should show up in the list.
If you're using a netlist flow then again edit the symbol and perform Tools - Update Symbols to populate the change list.
If this doesn't help contact customer support.
On your second question, you can copy/replace sheets by opening two sessions of DxDesigner, one on the design you want to modify ('target') and one on the 'source' of the new sheet. In the navigator of the source you select the sheet and use Copy from the context menu. In the target to paste the sheet in, select the block and use Paste on the context menu. Two things to note on how this works. If you wish to replace sheet x in your target with sheet x from the source ensure you select sheet x in the target and then use paste - this will replace the target x with the source x. If you want to copy sheet x from the source into the target but leave the original sheet x then don't have any sheets selected - when you perform the paste the imported sheet will be renamed to sheet x_1 to ensure no clashes of the sheet name