Forms :: Populating And Protecting Fields On A Form

I am displaying records from a table on a form line after line like a spreadsheet/data sheet.

I want to do 2 things.

When the user clicks anywhere on a new row to add data I want to populate that row with some data from the row above.

I also want to protect certain fields on each line (but not all lines!)

So basically if the user is presented with

old model number.....new model number..Added by.....date

123456...................77777777................. Fred..........15/10/2014 10:11:12
77777777..................8888.................... .John..........16/10/2014 12:12:12
8888..........................6767................ .....Jack..........16/10/2014 14:11:12
6767..........................4321................ ......Joe..........17/10/2014 10:11:12

I want to protect all of the 1st 3 rows and the old model number in the last row.

When they tab or click into the new row I want the old model number populated with the new model number.