If your existing KXTDA 100/200 is running version 3 software or later, it can easily be upgraded to a KX-TDE by replacing the existing main processor card. Systems running earlier versions of software will need upgrading to V3 before being able to be upgraded. The KXTDE operates on any standards based 10/100 mb network. Non IP phones will require a conventional wiring infrastructure.
Hmmm reading the site it seems a little restrictive in upgrades, and I wonder what the cost is to do the upgrades.
Re:Why should I choose elastix over panasonic kxtd
I am not for sure, But one of Panasonic reseller told me, That when You have TDE You need to
by (purchase) SIP accounts, Whit ELASTIX that is not case, You can expand your system however you like and your server able to accept. I think that is Elastix is probably chipper variant of making PBX, one think where I thing that you will fund troubles is FAX-ing which is build in Elastix but sip and voip still hard handle that stuff. My answer is based on approach where you making Your won server, Unfortunately I do not have experience with Elastix dedicated hardware yet.