For older HAG In most cases this is very easy assuming you are putting in a digital decoder. A marklin pickup shoe can work, but best to go OEM HAG.

You really don't need anything else unless you want to properly upgrade the lighting beyond the factory chassis ground system.

I certainly don't want to spoil the fun or discourage you, but your possible project depends on many specific factors:

- HAG produced HO models from approximately 1956 until today
- Generally, a conversion is possible, but there are enormous differences between the models/generations
- The original and "true" HAG company closed down in the year 2012
- Since then, a new outfit has taken over the business - I will refrain from commenting their quality and "business philosophy" for obvious reasons
- Yes, the wheels will almost certainly be a problem: the AC/DC wheel flange spacing is not identical, and often you can't just adjust this in a few minutes
- Giving you tips would be very much easier if you could indicate the type/item number/production year of a specific model you have set your eyes on
- Obtaining (new) models even from Swiss dealers is becoming increasingly difficult because many dealers no longer wish to deal with the new company mentioned above
- A list of dealers still willing to cooperate with the company is available here: https://www.hag.swiss/haendler/ (Swiss domestic on the left, international on the right)
- There is a HAG web forum that has been active for over 20 years: https://www.forum.hag-info.ch/ Communication there is in German, but English posters are welcome, especially when they use a translator like www.deepl.com In case of difficulties, I am one of the moderators there and can help.
- Good luck!

Not to further complicate, but remember if you are converting an AC unit to digital you will need to add the DC motor permanent magnet - which are expensive and difficult to source... (which is why I like converting the DC variants to AC...)

Not relevant if you are keeping it analogue of course.

Knowing it is green box-era, a DC to AC conversion should not be difficult at all...

Yes, unless somebody swapped boxes, this is an extremely reliable indicator of pre-2012.

BTW, there is also a HAG collectors database where you can search for all HO models produced between 1956 and 2012. Often you will find something easily by entering the "road number" ("Betriebsnummer") of the model (the printed number given by the prototype railway company to their rolling stock) in the database: https://frunum.de/ The Red Arrow in the picture below, for instance, carries the road number 604.

The Hag Shunter (Eem) was a product from Hag Classic (Tekswiss/Adolf Schoch), which used the Hag name for a short period after the closing of Morschwil and the original company. The name and business was later sold to Heinz Urech (Polytrona/Stansstad) who runs the official business today.
It all seems a little complicated and the new Hag (Stansstad) has no connection with the Tekswiss company.

To convert a modern (post-1985) model from AC to modern digital requires a few modifications. I have found that it is often better to find a digital model. Shops like Roundhouse, Hobby-Huesli and others may still have older models in stock. Acquiring those models can be a little complicated, especially if the shops don't want to ship them outside Switzerland. Converting Hag models was relatively easy when using the first generation Motorola 6080/6090 decoders, but when using ESU, Zimo or other, you have to make certain mods to the motor block and the wiring, especially if the model uses the chassis for the return (ground).

If you find a Hag model, there are places in Switzerland who will do the conversion or modify the model (new circuitry) if you send the model to them. It can cost a few hundred francs to get done, but if you are not an expert, it can yield better results.

Contact Luessi.ch, eyro.ch or other dealer for information on what services they offer (conversion, modification, etc)

Regards

Mike C