I was thinking about upgrading from hd500x to the  LT floor pedal its value is about £750 ,  I could probably sell the hd500x for around £270 , so I am just wondering what people think is it worth  upgrading  or a waste ,   
It's definitely worth it, even the LT is a considerable step up. Sound quality is greatly improved.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
I looked at both the LT and the regular Helix , bit the bullet and went with the Helix.

Aside from the fact that a lot of the ergonomics that make the Helix great to use are gone in the LT (and I'm not *just* talking about scribble strips), the construction quality is also very different. The Helix is pretty rock solid, and mostly made of castings. The LT version is folded metal and nowhere near as strong.

You'll also notice that the I/O is a lot different. They're calling it a "streamlined" hardware and I/O component and pushing it as an inexpensive backup for a Helix or Helix rack. Lots of "affordable" and "budget" references. There's just one FX loop on the LT (four on the Helix), one input for an expression pedal on the LT (two on the Helix), but that jack is also used for any amp switching you might want to do as well, so it can't do both. On the Helix, you have two separate amp switching control outputs. I don't recall if there's an Aux Input on the LT, but there is on the Helix, and the digital suite is missing some components on the LT (as I recall).

To answer the OP's question, however, "Is it worth upgrading from the HD500?" I'd have to offer a qualified yes. There are improvements in sound quality, but I wouldn't typify it as "greatly" improved (the HD500 is actually pretty good).

What makes a world of difference is the ability to use IRs.

That one thing will be the biggest change from the HD500 in terms of sound. I coupled my HD500 with a Two-Notes Torpedo C.A.B., which at $545 was actually more money than the HD500. What it offered was probably the best suite of IR cab sims around (with the ability to use imported IRs as well) and a set of tube power amp sims (EL34, EL84, KT88, etc., etc.). This was the area I thought the HD500 was a bit lacking, and the difference was absolutely worth the extra money. I would venture to say that the addition of the C.A.B. pushed the HD500 into competition with the high-priced spread across the board.

At that point, I'd have to flip a coin between the HD500 plus C.A.B. and the Helix LT. The LT might win based only on the routing options built into both of the Helix models. The extended I/O, the better quality construction and the ergonomic features (okay, including the scribble strips) *and* the routing options and any improvements on the native sims made the standard Helix a no-brainer.