Can you explain a bit more what the problem is?
Brompton changed the brake lever in 2008 from an so called "open clevis" to a "closed clevis" construction. The open clevis required nipples of a certain size on the brake cables. In case a non appropriate cable would be fitted to such a lever it could under bad circumstances work loose rendering the brake useless. In 2012 Brompton, in outlook to the new 2013 levers, changed the design of their standard brake cables that their nipple from thereon would no longer sufficiently fit the open clevis levers (bot both, the 2013 levers as well als the older closed clevis construction). As a consequence they released a product warning to their dealers about that fact and recommended that the brake levers on older bikes should be changed:
A running change will be made to a standard dimension brake cable nipple; which is shorter than the existing version used on our bikes. The two nipples are shown in the image below (Pic 1). (...)
This could cause a safety issue with bikes produced before September 2008, where the new smaller nipple will not be properly secured in the brake clevis and could result in the brakes not working. These bikes were fitted with a brake lever using an open clevis design; this is shown below (Pic 2).
The new cable nipple is compatible with both the newer closed clevis design (Pic 3) and the new 2013 brake lever (Pic 6).
The closed clevis was introduced in 2008, this safety notice affects bikes with serial number 0808XXXXXX or lower.
The affected bikes will be fitted with either 10 degree (Pic 4) or 30 degree (Pic 5) brake levers, depending on model. We recommend replacing these with the 2013 lever (Pic 6). Alternatively, the current 30 degree lever with a closed clevis may be used. However, a cable with the shorter nipple should under no circumstances be fitted to a 30 degree lever with an open clevis. THESE LEVERS MUST BE REPLACED.
You may be able to find the original pdf on the net - it's title is "Product_Change_Notice_Brake_Cable_Nipple_Final_v2_1.pdf", dated from
12/09/2012. The pictures in this document illustrate the issue better than words can.
So technically it is still possible to run and use the older levers, however, size does matter here and the newer constructions are clearly more safe towards a not so experienced mechanic whereas the older construction relies on a competent mechanic (which may not be a given and a hobbyist may potentially neither be aware of the problem nor the existence of different nipple sizes at all, let alone of the consequences).