I've only switched resorts twice, and both times were out of necessity.
First, was from a Hotel Plaza Blvd hotel to Music, because we weren't able to get our entire trip booked into Music. But unfortunately they don't handle transfers from non-Disney hotels, so we had to get a taxi and move ourselves.
The other time I went for 8 nights, but my friend could only go for the first 4. So we stayed at Riverside for 4 nights, he flew home and I transferred to Music to save money as it was just me and a Value room was fine by me.
That being said, if you're CHOOSING to split your stay, stay at the Value first, then upgrade to the Moderate. Otherwise, the "downgrade" is really veyr obviousl if done in the other direction.
Disney will help you with your bags. Just arrange it with Luggage Services. They'll tag your bags with special transfer tags and take them. In a few hours they'll be moved over. You can go play in the parks for the day. Then in the afternoon, take Disney transportation to your new hotel, check in, and call Bell Services from your room to have your bags brought up. (Remember to tip the Bell Services CM)
Also make sure that you link your flights properly to the reservations if you're using Magical Express. You'd have you arrival on the first one, and departure on the second.
Lastly, you'll have problems if you try and split your stay using packages, especially if you're getting the Dining Plan. You'd need to book two packages, each with tickets. It's best to book Room Only reservations, and buy tickets separately. The front desk can encode the tickets onto your first room keys. Then your second hotel SHOULD be able to take those tickets and move them onto your new room keys after you've checked in.
-Rob