Help! New mono cartridge for Dad's present-need the least amount of tweaking

There are a lot of ways to skin a cat.  To answer your question, yes you can buy him a DL102 and it will be very similar in weight and overall dimensions to his 103, therefore, it will be easy enough to switch back and forth.  All else being equal it should sound better too, because as a mono cartridge it is only picking up the signal in one plane off the record.  That means in theory it could/should be quieter.  In reality the difference is often negligible or very slight.  That is another way of agreeing with the guys who are saying why bother.  Another option would be to get him a summing switch which would sum the left and right channels into one mono channel electronically.  This does exactly the same thing as switching cartridges and is for preamps that don't have a mono button.  You can get such an item for short money from KAB Acoustics and it would be my recommendation.  Finally, if you are a real nutter like me you need two turntables....