As many answers here may imply too-far a fall from riches, perhaps:

Newly Middle-Class

Recently Reduced

If it was not a recent change to their status, I like:

Once-Wealthy

...as it gives a subtle alliteration that is not obvious in the spelling.

As many answers here may imply too-far a fall from riches, perhaps:

• Newly Middle-Class
• Recently Reduced

If it was not a recent change to their status, I like:

• Once-Wealthy

...as it gives a subtle alliteration that is not obvious in the spelling.