In many occasions when the design of the sites is changed for the old players they do not like it and they simply stop playing, I think that if the traffic of the site continues to give good results or stays the same, it is likely that things work well, such Maybe a massive advertising campaign can help, maybe many players like the traditional more, also the design they have seems to me to be excellent and maintains its authenticity.
It is analogous to changing the style of our forum, many say that it should be updated, but many are defenders of keeping the traditional, it is what makes it different from the others.
I do not think that you would have a problem when old players stop playing because I am 100% sure that there will be options available to continue with the old one, that is what thequin said, so that is not a problem. All of this is a mute point right now because thequin said that they are "considering" a new design, which is direly needed if you ask me, this looks like something a person slapped together 10 years ago and it is nearly exactly that, not 10 years but getting there, a brand new design with much better UI will definitely be better.
This is why I believe we are going to see a new design, and the old one will stay there as well, people will move to new one if they want to but if they do not want to they will stay at the old one and everyone will be happy that way, you are keeping the old customers happy and you are making new design wanters happy at the same time.