There are only two steps for some cases and three in one.
AC17 will open all files down to version 8.1 which was released in 2003
for prior to this version the converters will open
- AC10 opens up to AC 6.5 (1999)
AC09 opens up to AC 5 (1996)
AC07 opens uo to AC4.1 (1992)
Therefore to open a 22 year old file you will need three steps AC07 to AC10 to AC17.
AIA NCARBAC26 US/INT -> AC08
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Learning: VW, RVT
Teaching & Using: AC, Rhino, ACAD, RVT, SU, AI, PS, InD, TM, C4D, ATL