Hard to answer 
The elegia , in the focal lineup, is much like that weird brother.
No focal will share identity in the sound.There is the clear but despite the fact they both share the same driver they sound quite different
You should try if possible but i cant be categorical on if it s gonna be a an upgrade
I would say different.
If you can order them and send them back if you don’t like ,why not?
However the look of the focal celestee is definitely the best among all the focal.
If you want to stay on the focal course.
The (elex: open / elear:open/ celestee:closed)
Among the focal, i would say that the elex is a good twin companion 
Your question si tricky😅
On technical level the elegia is quite the innovation.
For being in possession of several open back headphone .
The elegia sounds almost open .
Especially in comparison of the wh1000xm4 and xm3.
As for the jump from mainstream to hifi.
I don’t want to hype up .
The main advantage is principally the latency… it s gone. ( from Bluetooth to wired)
You might be disappointed at first… the mainstream headphones are designed to be pleasing.
What you should look up in priority instead of branding and mainstream or hifi.
Is the frequency response.
Is it (v shaped )( neutral)( diffuse field)
Out of all the headphone i possess ,the hd600 is most literal “hifi”
Sounds more true to the source
If you want to crack the boxer ( etymotic er2se and hd600 are literally hifi)
The rest is compromise between fidelity and coloration)
The elegia is a little colored but lean more on the hi-fi spectrum then fun .
While having it’s fun trick which is to simulate an open sound.
Ho really revealing.
You will rediscover soundtrack with the elegia. Especially coming of the wh1000xm4 and airmax
There is some much more to say and it s hard to be concise… English is not my native language