Whilst it is generally desirable to change all older lights to LED, and to do so in one go, there can be times where budget and time prevent such an approach.
In such a case, it is desirable to prioritise and "get the most bang possible for your buck".
How can this be achieved?
1. Urgent safety related -
If broken, change immediately.
Wherever possible change similar lights in the same room or section at the same time - chances are they are all old and will all soon need attention. You do not want to pay repeated electrician call out fees as it costs less per light when the electrician has a full day's work on site.
2. Lights which are high up / hard to access -
Some lights need a cherry picker or scaffold tower to access. Where this is the case, changing one light can costs £ hundreds. If one such light fails, why not sort the whole site with high quality LEDs in one go. That way, you only hire the cherry picker / tower once and the electrician's time is used more efficiently.
Note:- DO NOT SKIMP ON QUALITY FOR THESE LIGHTS choose excellent and with luck it will be many years before any of them need attention again!
3. Lights which are on a lot -
The more hours a light runs, the quicker your investment will pay back. Lights running continuously 24/7 or 12/7 should be high priority for change.
If your area needs continuous illumination, use standard LED lights for around 70% electricity savings.
For areas with an intermittent requirement like corridors, stairwells, and toilets, microwave sensor versions are recommended often pushing savings to >90%.
4. Rooms which are too hot -
My previous company replaced halogen spotlights with LED in a shop which had previously used air conditioning well into late autumn. After the change, the air conditioning stayed off most of the summer!
Lights which do not fit in with any of the above are still usually worth changing unless they are very seldom used e.g lofts and rarely used cupboards, however can reasonably be left for later if the budget is tight.