Software engineering is defined as the application of the systemic, disciplined, quantifiable approach to the development, operation, and maintenance of software. It is the application of engineering to software. The classic life-cycle paradigm for software engineering includes: system engineering, analysis, design, code, testing, and maintenance.

Maintenance is needed to ensure that the system continues to satisfy user requirements. The system changes due to corrective and non-corrective maintenance. According to Martin and McClure, maintenance must be performed in order to:

• Correct errors;

• Correct design flaws;

• Interface with other systems;

• Make enhancements;

• Make necessary changes to the system;

• Make changes in files or databases;

• Improve the design;

• Convert programs so that different hardware, software, system features, and telecommunications facilities can be used.

Accordingly, software must evolve and be maintained.

This paper addresses the maintenance portion of PAT life-cycle. There are several improved architecture models presented witch make maintenance and evolution of PAT easier. Among the presented architecture models there is one model (with five layers) witch was adapted to the system. The new system’s efficiency was compared with the old one’s.

This paper also contains a new architecture model witch could be adapted to the new systems. The main purpose of this architecture is to separate graphical user interface from system’s business logics and make software maintenance very easy in the future.

Raktažodžiai: models, legacy systems, informatika, maintenace, priežiūra