In general all Original P1, Revised P1 and P2 panels that fit the standard 5.75" deep x 20" wide enclosures are interchangeable within the same height of enclosure and can use the same Type 1 fronts. 

If original installation fit a 44" high enclosure - any P1 or P2 interior that fits a 44" high enclosure will fit same enclosure.

Example 1:- Existing Original P1 interior is in a 5.75"x20"x44" enclosure - this includes the subfeed space:  
ref: P1C42FX250ATS
Options for replacement:
a) RP1 interior with subfeed space that fits the same 44" high enclosure - P1C42FX250ATST  - may be able to use UPB interior and move branch breakers to new interior.- Main Breaker or MLO kit cannot be re-used.
b) RP1 interior without subfeed space that fits the same 44" high enclosure - P1C54FX250ATSN  - 12 ADDITIONAL CIRCUITS - NO SUBFEED SPACE - Factory assembled only - Branch Breakers could be moved over if ordered with provisions.
C) P2 panel that once configured fits the same 44" high enclosure. Factory assembled only - Branch Breakers could be moved over if ordered with provisions.

Example 2:- Existing Revised P1 interior is in a 5.75"x20"x44" enclosure - this includes the subfeed space:  
ref: P1C42FX250ATST
Options for replacement:
a) RP1 interior with subfeed space that fits the same 44" high enclosure - P1C42FX250ATST - same as original
b) RP1 interior without subfeed space that fits the same 44" high enclosure - P1C54FX250ATSN  - 12 ADDITIONAL CIRCUITS - NO SUBFEED SPACE
c) P2 panel that once configured fits the same 44" high enclosure.

Example 3:- Existing P2 interior is in a 5.75"x20"x44" enclosure - this includes the subfeed space:  
ref: P2C42QR250ATS
Options for replacement:
a) RP1 interior with subfeed space that fits the same 44" high enclosure - P1C42QR225ATST
b) RP1 interior without subfeed space that fits the same 44" high enclosure - P1C54FX250ATSN  - 12 ADDITIONAL CIRCUITS - NO SUBFEED SPACE
C) P2 panel that once configured fits the same 44" high enclosure.