Did this make sense when you typed it?
They have a signed agreement (the PUD). This means the city agreed to this as part of getting the developer to come in, in good faith, and build within the agreed upon zoning for the entire project covered under the PUD.
This development has brought millions of dollars already into the City of Somerville, hundreds of new housing units, hundreds (probably thousands) of new jobs including a massive anchor tenant in Partners Healthcare, a new mass transit stop on the orange line, and really helped kickstart new economic development in a City during a pretty tough time (remember the climate back when this thing broke ground.)
Now the city wants to renege on the agreed upon deal that brought this golden goose into their city. The PUD has been amended a number of times over the years (some give and some take based on the time frame), but now the city wants to forgo that process, and just say "thou shalt". F that. I hope the state slaps them down just like they did for Northpoint mentioned above. Make future additional parcels not covered under the original PUD meet the new standards, because they are now law, but those laws cannot be retroactively applied to previously zoned parcels under the PUD process.
This is a purely greedy move by the City who keeps wanting to put on it's big boy pants, and think its got that Boston clout. Nope. Just because we're now at the tip of a great building cycle, you don't have the power to now extort a developer who helped carry you on the upswing of that building boom, just to then ride them down it on the backside.