waqas.sabir wrote:my only concern is I have read somewhere that if applicant has changed immigration category then 3 month concession will not be applicable..
That's not quite correct. The 3 month concession only applies to those whose 5 year residency period began with an Economic migrant EC visa (this could be in any of the work visa immigration categories which lead to ILR). The reason is the concession is given for delayed entry into the UK.
The concession doesn't apply to those whose 5 year residency began by switching from a non-economic migrant immigration category into one because Entry clearance doesn't apply here, i.e. because you can only switch immigration categories while you are in the UK. E.g. PSW to Tier1/Tier2 (in-country).
If you entered the UK on your WP EC then this 3 month concession will apply to you. Additionally the fact that you switched to Tier 2 after that will not be an issue.
waqas.sabir wrote:Is premium service best option or post service I have spoken with a solicitor he is saying postal will be safe because by the time they will start working on the case the gap will become more less (gap between EC and travelling date is approx 60 days at the moment).
IMHO, you have nothing to worry about. You can apply either in-person at a PEO or by post as you wish.
See CW guidance for further info:
http://www.ukba.homeoffice.gov.uk/sitec ... iew=Binary
Applications that fall short of the five year continuous period
In some cases, applicants may have been granted five years continuous leave, but due to delayed travel will not have spent five years continuously in the UK before their current leave expires. Caseworkers may count the period between entry clearance being granted and the date the applicant entered the UK towards the five years, provided this period was not longer than three months. 
