IFRS 15 replaced IAS 18 Revenue, IAS 11 Construction Contracts and some revenue-related Interpretations. It introduced major changes to the revenue recognition principles. Here are some key changes brought about by IFRS 15 compared to previous standards:

1. Five-Step Model for Revenue Recognition: IFRS 15 introduces a structured five-step model for revenue recognition, which is more comprehensive and prescriptive than previous standards.
2. Performance Obligations: The concept of performance obligations is a key change. Revenue is recognized as performance obligations are satisfied, not necessarily when cash is received. This was less clearly defined in previous standards.
3. Variable Consideration: IFRS 15 introduced more detailed guidance on how to deal with variable consideration and constrains the amounts that can be recognized.
4. Timing of Revenue Recognition: The rules on whether revenue is recognized over time or at a point in time are more clearly defined in IFRS 15.
5. Contract Costs: IFRS 15 provides specific guidance on accounting for costs to obtain or fulfil a contract. Some of these costs may now be recognized as assets, where they previously would have been expensed.
6. Disclosures: IFRS 15 requires more detailed disclosures to provide users of financial statements with comprehensive information about revenue.
7. Principal versus Agent Considerations: The new standard provides more comprehensive guidance on assessing whether an entity is acting as a principal or as an agent.
8. Contract Modifications: There's a detailed guidance on accounting for modifications to contracts with customers.
9. Licenses: IFRS 15 contains specific guidance on recognizing revenue for licenses of intellectual property.
10. Customer Rights: The treatment of customer rights, such as warranties and refunds, are more clearly defined.

These changes were made to address inconsistencies and weaknesses in existing revenue standards and practices, improve comparability of revenue recognition practices across entities, industries, jurisdictions, and capital markets, and simplify the preparation of financial statements by reducing the number of requirements to which entities must refer.