This is a creative pastiche piece that applies Chopin's characteristic harmonic language to a well-known birthday theme. It blends recognizable melody with sophisticated contrapuntal rigor.
The challenge lies in applying Chopin-esque nuance and rubato to a tune usually played with rhythmic rigidity.
Comparable difficulty and character — good candidates for what to learn next.
Canone minore alla Chopin su 'Happy Birthday to You' is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Canone minore alla Chopin su 'Happy Birthday to You' lasts about 2 minutes.
Canone minore alla Chopin su 'Happy Birthday to You' is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include The Entertainer by Scott Joplin, 12 Variations on "Ah vous dirais-je, Maman" by Wolfgang Amadeus Mozart, Piano Sonata No. 11 in A major by Wolfgang Amadeus Mozart — see the full similar-pieces list on this page.