I think you want the decision table to return the new number if it's fed with the old number and to return the old number if it's fed the new number.
No, you cannot use a decision table in a bidirectional manner because there is a clear separation between condition columns and result columns of the table. Condition columns represent the input to the decision table and the result columns represent what the table will output. That means either you can have the old number as the condition column and the new number as the result column or the other way round.
If you really want to make do with just one decision table, you could create one table with a condition column called something like "Input" and a result column called "Output" and you could maintain both the old numbers (in the "Input" column with their corresponding new numbers in the "Output" column) and the new numbers in the table. But, that's simply merging two tables into one and would work only if there are no overlapping values.
