The situation has changed significantly with the introduction of EASA Part FCL. This summarises the requirements of the new licence,
1. For a start, there is no requirement to hold a current rating to keep the licence current. In fact, the licence is valid for life, providing you can keep your medical.
2. EASA ratings are only valid for a year.
3. Your EASA licence has a section for previously held ratings and any expired ratings will require enough training as deemed necessary by an Authorised Training Organisation. If you are current on a type on another ICAO licence, then this will probably simply entail taking a Licence Skills Test (LST).
4. If you need to put another ICAO type on your licence not previously held, then see the answer provided by Strewth. You will require a letter from your current employer showing that you are in current practice and stating hours on type, in addition to a course completion certificate to show you have completed a recognised groundschool.
5. A prospective employer may be persuaded to do the LST for you as they will have to do a Licence Proficiency Check (LPC) anyway and the only additional item is a raw data approach.
6. If in the position of having to have an EASA type rating to start the job, this is not insurmountable. Get the letters from the company and get in touch with an ATO who can issue the type rating. You will have to use an ATO, using a mate who is a TRE is no longer acceptable. But it's certain your mate works for an ATO, so he can probably organise it.
7. If not planning to use the type rating, then there's no point in getting it and/or keeping it current as they only last a year and have to renewed by an annual LPC in an EASA approved sim with an EASA TRE.
In addition, it's useful to know what to do if you currently hold a JAR type licence. It will have been issued for 5 years, which means the last to be issued will expire in about another two. The EASA licence can be issued with any of the agencies, but the definition of which one is where your medical records are held. Each agency follows PART FCL, so in theory, there should be little difference between them. However, practice has shown there are subtle differences. The UK CAA transfers your ATPL without question on submission of the appropriate form and fee.
However, the one sticking point has been the ICAO language proficiency. They won't accept the proficiency level on your HK licence as it wasn't assessed by an EASA assessor. You probably have 'Proficient' on your JAR licence which essentially means level 4 and which lasts 4 years, so it will probably have expired. Some language schools used to do English tests on Skype, but the CAA don't accept these as from April 1st, as it appears there has been some fraud! The form does say that the British Council can recommend an approved school to do the test face to face, but when I contacted the BC in HK, they of course knew nothing about it. They are in contact with the CAA for guidance, but as of now, there is no way of doing the test outside Europe. Another way is to get an EASA examiner to assess you during a test. Any TRE can do this and sign you off providing he is level 6 himself, so this can be done as part of your LST.