They are not allowed to take you old (original) licence from you, and will normally make a copy of the documents.
The original licence is in most cases still a valid licence for you to drive in your home country, it is also accepted in most countries as a legal form of identification, and in some countries is a government document and as such may not be handed over or used as “collateral”.
The process of conversion involves a set of ID photographs, your passport, you need to present your original licence, but they will only take a copy of it – you need to re-take an eye examination.