Data-only-locked tables require more overhead per page and per row than allpages-locked tables, so the maximum row size for a data-only-locked table is slightly shorter than the maximum row size for an allpages-locked table.
For tables with fixed-length columns only, the maximum row size is 1958 bytes of user data for data-only-locked tables. Allpages-locked tables allow a maximum of 1960 bytes.
For tables with variable-length columns, subtract 2 bytes for each variable-length column (this includes all columns that allow null values). For example, the maximum user row size for a data-only-locked table with 4 variable-length columns is 1950 bytes.
If you try to convert an allpages-locked table that has more than 1958 bytes in fixed-length columns, the command fails as soon as it reads the table schema.
When you try to convert an allpages-locked table with variable-length columns, and some rows exceed the maximum size for the data-only-locked table, the alter table command fails at the first row that is too long to convert.