DNS Cache and your ISP:

There is also the issue of DNS cache, which is something we won't go into great detail about here, but here's the short version. 

Every time you access a site from your ISP, they cache the URL, as well as its associated IP number. 

If their network is properly setup, these DNS cache records should "Expire" at least every 24-hours. 

If they did not (which is often the case), you'll experience this: 

You enter your http://www.mydomain.com/ URL, and it keeps taking you back to your old server account.

In a large number of cases, it's the result of an ISP who "Did Not" configure their servers to "Expire" the DNS cache records at the appropriate intervals. 

Unfortunately, this adds additional confusion to their clients, and especially the ones whom are trying to point their domain name to a new server. 

The DNS propagation process is not limited to ISP's!

The Internet itself must update/clear its DNS cache as well. 

When we say the Internet, we mean the numerous intermediate "points of access" you're routed through before reaching your final destination. 

For the most part, these intermediate points of access consist of "Internet Routers" and "Internet Caching Engines." 

These too, maintain their own DNS cache, which assists them in routing traffic/resolving URL's to the correct destination IP's. 

Don't worry though, as Internet routers are usually faster at clearing their DNS cache than ISP's are.

What to expect during this 2 to 4 day propagation period:

In most cases, the propagation process will take at least 48 hours to complete. 

The first thing that happens is the "World Root Name Servers" will check all of the various "Domain Registers for updates.