The old vesting plan is unusual, the new vesting plan is much more common. I'd suspect that the company had an unusual vesting plan for some unusual reason and now that the company is getting more mature and "normal", the rationale for that unusual vesting plan has gone away.
For example, consider an early company that's several years away from having a product ready to hit the market. Having key employees leave after 18 months might harm that company more than a more mature company with a product already on the market. A company that was very dependent on long term plans or a few key employees might prioritize not losing any early employees for the first three years. However, once the company matures, that unusual requirement might go away.
The new vesting plan is better for employees in that they start getting some of their stock after just one year. Being terminated after 20 months and not getting any stock would kind of suck. To some extent it's worse for them since they don't get all of their stock until four years elapse, but they can also negotiate for additional grants during that last year if that's appropriate.
It's likely better for the company as well because it still gives them one year to terminate a poorly-performing employee without granting them any large chunks of stock and avoids hard feelings if they terminate an employee during the second year. Also, if they're considering additional funding rounds or acquisitions, it helps to have a more normal employee vesting plan.
I would take it as as sign that the company is becoming more mature and normal and less worried about unusual aspects of its early development.