I don't see it taking less than four years, realistically. You're basically asking for a full remastering. A not wholly inaccurate comparison would be WoW Classic, and that was a less ambitious project working on a smaller content base that still took somewhere in the realm of 3-4 years.

The only reason ARR worked in the time it did was because of a willingness to commit to an absolutely unreasonable pace of work, and because they were recycling the bones of a game that was literally months old when planning began. An engine 'update' presumes keeping a decade of content wholly intact and functional and that is, paradoxically, a much more challenging ask. It's also the kind of project that is hard to shave time off of by just adding more people - feature-heavy, high-complexity projects are prone to being riddled with volatile bottlenecks where one person's work can hold up dozens more from even beginning theirs.