In upgrading a concentrated executable computer code with an upgrade code, indexed lists are created for the concentrated code and for the upgrade code. The indexed lists have index references for code identifiers of code structures in the concentrated code and the upgrade code, respectively. The indexed lists are stored in a virtual table. The concentrated code is integrated with the upgrade code to form an integrated code. The virtual table contains an operable part referable by the integrated code such that when a code structure being executed originates from the concentrated code the indexed list for the concentrated code is stored in the operable part of the virtual table, and when a code structure being executed originates from the upgrade code the indexed list for the upgrade code is stored in the operable part of the virtual table.