If you don't change your underlay when you change your carpet, there are several potential issues that can arise:

1. Reduced cushioning: Over time, underlay can lose its cushioning and support, which can lead to a less comfortable and less durable carpet. If you reuse an old, worn-out underlay, it may not provide adequate cushioning for your new carpet, leading to premature wear and tear.
2. Reduce the life of your carpet: An old underlay may have indentations or worn areas that can cause your new carpet to wear unevenly. Additionally, if the underlay is not properly secured or flat, it can cause ripples or bumps in the carpet that can lead to tripping hazards and potentially damage the carpet.
3. Poor indoor air quality: Over time, underlay can accumulate dust, dirt, and other allergens that can affect indoor air quality. If you reuse an old underlay, you may be re-introducing these allergens into your home, which can be especially problematic for individuals with allergies or respiratory issues.
4. Reduced energy efficiency: A quality underlay can help insulate your home and improve energy efficiency. If you reuse an old underlay, it may not provide the same level of insulation as a new, properly installed underlay, leading to higher energy bills.

In general, it is recommended to replace your underlay when you replace your carpet. This will ensure that your new carpet is properly supported, cushioned, and insulated and can help extend the life of your new flooring. In addition to this, it is important to note that most carpet manufacturers state that you must change your underlay, or it will void the warranty on the carpet. For more information on your carpet warranty, please contact your retailer.