reissue train tickets

couldn't spot that individual offer, but an educated guess based on any railway-discounts across Europe nowadays: special deal tickets cannot be refunded (as compared to standard fares), so if your travel plans change, your ticket is void. that what it's likely all about.

The discounts are for all national train lines.

Reissue means "revalidate", that is, you can purchase the discount tickets always as long as the new journey is at least 5 days away (including the purchase day and up to 60 days) from when the ticket is reissued.

If you are not sure about your dates of travel don't buy tickets in advance. Train travel in Portugal isn't expensive and you can get a ticket every time before you get on the train (possibly with only exception of popular holidays).

But if if you're sure of where you going to, buy it.

"Travel in Portugal isn't expensive" - For whom? 40% is money.

The best normal ticket available from Lisbon to Porto costs 42,40€, with the discount ticket it's 25,5€. You save 16,9€, enough for a meal for two!

I would interpret that warning as nautiker does. It is telling you that if you wish to change your ticket in any way, you will have to pay for a new ticket. Just as discounted airfares often say, 'non-changeable, non-refundable' etc.